No. Not only is the Cram101 series not useful for learning, but it's a scam. Here is a review of the "outline" for Billingsley's Probability and Measure:
I thought it was some comments on the Billingsley's book, Probability and Measure, but it is just a small notebook with a kind of a (very bad) dictionary of probability terms in the margin of each page. The idea is that you do not need to highlight your Billingsley book, but copy the passages to this notebook instead. I put it in the trash already. Don't buy it; this is not a book!
Another, for an unnamed statistics book:
I just received my "Cram101" book in the mail, for a graduate level statistics course. It is entirely definitions, for words such as "theorem" or "linear", and has nothing to do with the course. It is only 54 pages long, and all of the right hand pages are blank. The final page even defines "rat" as being a "various medium sized rodent". Wish I had read these blogs before I purchased.
Now I personally don't have access to any of their math books, nor a desire to flush \$30 down the metaphorical toilet. As a substitute, I found out that a local university library had a few of these "outlines" for texts in other fields (I sincerely hope that these were "donated" and not procured using library funds). Here's what the content pages look like:
The right-hand pages are ostensibly for you to write your own notes.
Leafing through the "outline" for Varian's Intermediate Microeconomics, I can confirm the running themes of reviews of Cram101 publications:
The terms don't match those of the text. A full 75% of the bolded terms from the chapter on game theory (such as "payoff matrix" and "prisoner's dilemma") are missing in the Cram101 rendition. Instead...
Terms appear over and over again. Don't make me count how many times the definition of "economics" appears.
Many terms and definitions have little to do with the topic. Highlights include
Argument | The discussion by counsel for the respective parties of their contentions on the law and the facts of the case being tried in order to aid the jury in arriving at a correct and just conclusion is called argument.
Context | The effect of the background under which a message often takes on more and richer meaning is a context. Context is especially important in cross-cultural interactions because some cultures are said to be high context or low context.
Continental Airlines | Continental Airlines is an airline of the United States. Based in Houston, Texas, it is the 6th largest airline in the U.S. and the 8th largest in the world. Continental's tagline, since 1998, has been Work Hard, Fly Right.
The definitions in fact appear to be scraped (and plagiarized) from elsewhere. For example, the definitions quoted above can be found respectively at here, here, and here. I suspect that the terms and definitions are shoddily compiled from the internet by some algorithm (here is a similar allegation), and really, that's the only way they could possibly produce 20,000+ volumes of these "outlines" without bankrupting themselves. Additional documentation of Cram101's practices can be found for example here.
Conclusion: Don't give them your money. You'd be lucky to find even one formula in the "outline", and even more so to find one that's actually from Stahl's book.