I originally had all the concerns about DRM, eventually obsolete formats, inability to lend, not supporting local booksellers....but after getting an e-reader, they've pretty much all been trumped by one thing: I can set the book down and have the damn thing lay flat and readable no matter where I'm at in the book. No more trying to find something heavy to set on one side or the other of a hardcover until I'm within the middle 40%, no more setting a paperback facedown any time I want to use both hands at lunch. Almost all of my book-reading time these days seems to be while I'm out at lunch or dinner, so it's a big improvement for me. I think this feature also contributed to finally getting me to start going to the gym (at least occasionally), as I can still get some reading done there.
I've also enjoyed the space savings of not needing the physical books around, and the ability to purchase new books from home and have them immediately available, but lay-flat is the top reason why I frown a little any time I want a book and can't find it in e-book format.
Books-on-tape might be even more convenient for my situations, but the idea of being forced to read at someone else's pace just makes me shudder.