I teach composition/rhetoric at a two-year college and I have required the previous (second) edition of this book for a couple years.
I chose this text initially because, unlike a lot of composition texts, the writing voice in TS/IS is very personable, easy to follow, and unpretentious. It occasionally shows a nerdy sense of humor which, surprisingly, my students don't seem to mind.
The book addresses issues that I wish I had been taught as an undergrad, like "saying why it matters," or even the concept of "they say/I say" itself, which looks at how to take empty, boring writing, and put it in context so readers are interested in it. Instead of lecturing about the same tired old essay structure, the book approaches the task like a personal quest: make yourself clear, be interesting, and have integrity as a writer. Anything it omits or only touches on, like MLA style or research tips, can easily be supplemented.
The publisher's price on the second edition (without the readings, many of which can be found online anyway) remains an affordable $25. And I was pleased to see this new third edition also at $25 (on the Norton site, anyway). The price allows me to pair this required text with another inexpensive book on style to capture the more technical side of writing.
My only disappointment is that while TS/IS is FINALLY available as an e-book in the new third edition, it is limited in platform, and comes with this disclaimer: "Our eBooks are Flash based and will not work on the iPad, iPhone, iPod Touch, and some Android devices."
So... my students can buy the cheaper e-book for only $15, but they can't view them on the majority of personal devices, which chips away a little at the advantages of the e-book in the first place.
Overall, though, the quality of the advice, content, and style in this book are still better than most texts I've worked with, so I will stick with it for the new year.