I used the iPhone's "Kindle" app to purchase and enjoy My Revolutions, the first fiction I've read in a long time. I read a lot of scripting manuals (but do very little real development) and I read about 3000 pages of baseball predictions and notes a year (sadly, this is not an exaggeration). The book was excellent!
Once I settled on a font size (that sounds ridiculous) the text was very easy to read, at home, on the subway, or before going to sleep. I did have some pretty severe Kindle envy on the B train tonight, but I regained my focus quickly.
I have one major complaint: I found the act of finishing the book unsatisfying; after paging through 4393 "locations" (the Kindle doesn't entertain the conceit of page numbers) I missed the satisfaction of closing the volume one final time. The application resisted my impotent flicks, always returning to the final page.