Kindle as a Tool for Mathematicians? UPDATED: 1/15/2014
I originally wrote this post in 2010, when I was looking for alternative ways to store and transport papers. I had my laptop, but due to its weight, limited battery life, and the LCD screen, an e-reader such as Kindle seemed like a good idea at the time. (Also, at the time of the original post, I had never owned a smartphone, let alone a tablet.)
Aside from the Kindle, are there any other electronic tablet or pads, or other devices that you'd recommend for this type of purpose? What are some of your experiences?
As for my self, I purchased a Pocketbook Reader 602 from PocketBook International four years ago. At the time, it seemed like a smart purchase, as it PBR handles a while array of file formats without needing to convert anything. However, I don't use the device as nearly as much as I thought I would. 
The device itself works the way it should. It is a bit slow with the page rotation. And as user641 points out, the PBR can be a bit sluggish with larger files. The text-to-speech feature is completely useless when it comes to reading math. I tried utilizing the internet connectivity. While it is amusing to see websites in e-ink, in the end it's too sluggish to be of any use.
Here are a few things that I had originally thought would be convenient, but wound up being an annoyance instead.


*

*Lack of Touch Screen Capability. I purposely chose the model that didn't have a touch screen capability. Reports of glare and the idea of getting smudge on the screen led me to that. However, the alternative is an extremely painful, unintuitive, tedious navigation.

*Small Size. I opted for the small size of PBR 602 for its portability. While I have no problem reading novels, for math this is just unbearable. The slow page turn / search interface / zoom makes the problem even worse. Basically, reading anything that requires jumping from one part to another, looking up index or keywords, is extremely painful. 
Note that the small size wouldn't be as big of an issue, if the interface was quick and seamless. For instance, I don't find reading math on my smartphone as painful.
I still use my PBR 602 from time to time. In fact, I go through periods where I would use it extensively. Unfortunately, its inconveniences prevents it from regular usage. 
 A: I will share my experience here.
At first I also bought a Pocketbook e-reader, specifically the 902, which has a larger screen.  It handled both DJVU and PDF files, which was important to me.  The DJVU rendering was slow, however, and it had serious lag issues with large files.  But all the math was perfectly rendered, zoom worked, etc.
I eventually got rid of my Pocketbook because I found the device somewhat unreliable.  It was also clear that reading DJVU files on an e-reader is not the joy you might think it is.  Specifically, these are normal (A4) sized pages, being shrunk to fit the screen. I downloaded a few programs to help with the margins, but still, it was difficult/not worth it to read DJVU files this way.
I then purchased a Kindle DX, which can handle PDF files, but not DJVUs.  The Kindle is an incredible product, and works very well.  Still, it will lag on larger PDF files, and several times I have had to trim margins on my laptop before putting them on the Kindle.  This is really nothing to do with math however: I don't think there are any e-readers out there which handle PDF files like this well.
Now there a couple of options.  One is to convert the PDF files to an AZW (or even MOBI) file.  In my experience, this fails miserably.  Even without any formulas/math type, the spacing and sentence structure is essentially ruined.  With math? disastrous results.  And if there are pictures, forget it.
The other option, which isn't always available, is to get your hands on the TeX source.  I use this option for any arXiv papers I download, for example.  There you can resize the font, margins, etc., to optimize for Kindle viewing.  There are measurements out there which give exact dimensions for the viewing window of a Kindle. This option (which again, isn't always a possibility) is certainly the best.  Reading a paper on the Kindle this way is just as enjoyable as reading a hard copy.
In the end, the books I do have as PDF files I trim myself (using pdfTK and a Java program called Briss), and then put them on the Kindle as PDF files.  I view it rotated 90degrees, because this increases the magnification.
The articles I can get as TeX files, I adjust the margins/font size etc. (this is based on the paper itself; for example, large figures require different processing than a short, all-text paper). Then I convert to PDF and put on my Kindle.  Again, these are the nicest PDFs on my Kindle.
Finally, I would like to mention that several prominent math books (a few by John Stillwell for example) are now available in Kindle format on Amazon.  I haven't purchased any of these, but presumably the publisher simply changed the margins/text size in the TeX file, as I do for the articles.
A: I have the previous-generation Kindle, though I think the software and screen are essentially the same.  I think PDF would probably be the only convenient format and letter-size PDFs are a bit small to read without having to zoom and pan.  Personally, I wouldn't want to deal with a textbook on it—in fact, I wouldn't want to deal with textbooks on any tablet device I've seen thus far because you lose physical features like looking at two books at once and flipping through the book—but I'd certainly consider using it for articles or other shorter works.
A: The link https://mathoverflow.net/questions/30511/ebook-readers-for-mathematics really answers my original question.
I wanted to say that I ended up purchasing the Pocketbook Reader 602, and I've been using it to read math papers and textbooks.  There have been some issues with the firmware and wifi connectivity.  I actually had to send it back because of a broken reset button. The bluetooth file transferring interface and program could be a bit better. But overall, I'm very happy with the device.
http://www.pocketbook-usa.com/support/pocketbook-602/
Some of the complains my friends had were that it wasn't touch screen and that the screen was too small.  I actually did not want a touch screen, and I opted for the smaller device for the portability.
I personally have no problems with the screen size. The only times this ever became an issue was when the file had too much white blank space. I used a pdf editor and cut out the white space, and now it's totally readable. I have a lot of djvu and ps files, and so far PBR has had no problems handling them without having to convert them.
EDIT: When I first made the purchase, I was somewhat disappointed by the flimsiness (the reset button broke off) and issues with the software (bluetooth working sporadically, wifi issues, inaccurate battery meter), the page loading time was a tad too slow, the text wasn't dark enough, the automatic screen rotation was not working, etc etc etc...
Yes, it is a good e-reader, and yes, I think at the time I made this post, I think it was better than some of the other e-readers I've seen being used for math. But ultimately it was more of a toy than a tool. The device had been collecting dust for most of the part. 
But, then there was a firmware update.
The update fixed most of the issues I mentioned above. I haven't really used the wifi, but the bluetooth now works reliably. The overall performance of the device was greatly enhanced. Page turning is a lot faster, the text is clearer and darker. And the battery meter seems to be working in order too.
Now I use this device seriously, and it has been of a great use. The new interface looks a bit cartoony, but it is a lot more user friendly.
A: Following suggestions in an MO discussion of "tablets" good for math, I spent a few dollars and acquired "GoodReader" and "iAnnotate PDF" and "Note Taker HD". I already had "Pages" on my iPad ("1", not "2"), as well as iBooks.
With the "zoom" capability, they're all at-least-ok for reading PDFs. The iBooks is obviously not really aimed at this application. The "GoodReader" has functionality similar to the parts of Adobe Acrobat Pro that I use on my desktop Mac to mark up drafts (and other), in touch-screen form. Easy to organize files, too, at least in the sense that the interface for doing so is consonant with other interfaces familiar to me. The "Pages" creation of PDFs is not so much what I use, tho' it is the only way I can really produce PDFs on the iPad. I've not had occasion to compare iAnnotate yet.
The "note-taker HD" is quite striking, tho' I've not yet tried using it seriously. I do intend to attempt some note-taking (writing with my finger!): the resulting PDF file can be emailed, scp'd, etc., rather than needing photocopying... even if one eventually retypes-and-discards it.
In summary, despite the relative tininess, zoom makes it hugely better than lugging stacks of paper, to begin with. A very small laptop might compete, if it had "zoom", to make it worthwhile carrying the extra pound of weight... In fact, tablets fit under airplane seats much-much-much better than even small laptops. Battery life is much better, too.
I intend to try taking notes with the "note taker HD" sometime soon, to gauge the feasibility of writing as fast with my finger (stylus?) as with a pen. Obviously some of the issue is the feedback loop.
I believe that if one acquires the correct cable, that wide-plug from an iPad can run a TV/projector (tho' of course the battery life will be worse). 
The combination experiment, which I may perform this coming academic year, is using the mark-on-it-with-your-finger possibility, together with the projection possibility, to give lectures via iPad. My previous objection to "projector" talks has mainly been the static-ness (at least as the state most speakers default into) and non-adaptibility. Being able to scribble and high-light a projected typset screen, as well as "writing in the margin" (with my finger...or stylus) may allow a sufficiently dynamic version...
... not to mention allowing preservation of the marked-up version as a PDF, made available on-line for students, etc.
That is, an iPad with a teensy further investment in "apps", at least if one has a Mac machine to sync it with, seems quite excellent given the trade-offs.
Edit (14 Oct 2012): Used "Note Taker HD" all last year, 2011-2012, and it worked pretty well, although since I was just marking on PDFs I didn't get around to using the "zoom" feature to make small, precise marks. Since then, I've experimented further with various further possibilities: Penultimate (no small writing, but trivially easy to use), Notes Plus (pretty-good small writing), Notability (acquired recently), and just now, at the urging of hhh, "UPad". As suggested by hhh, the "small writing" (a.k.a. "zoom") in UPad is somewhat better quality than the others, insofar as it remains equally responsive to stylus pressure, etc., rather than losing that granularity of control.
I note that a somewhat different style of writing/note-taking seems necessary/better with these devices rather than paper-and-pen/pencil, namely, that color-emphasis, highlighting, cut-and-relocate, and "undo" change the ground-rules significantly. I do not claim to have figured it out...
The possibility of making a recording (of someone's talk) syncrhonized with note-taking exists in many of these apps, but there is the complication that most speakers do not anticipate that anyone is recording them, etc., either in terms of intellectual property or "attributable quotes", so I've not touched that.
A: For e-Ink, and math documents, you either want a larger or better screen.  Curiously, Kindle no longer uses the highest resolution screens.  That honor now goes to the iRiver Story.
