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.