Pretty much everything everyone has already said.
The biggest pluses for me:
1) Saves space! (I hate clutter)
2) You can easily prop the reader it in any position. I can read on my side in bed and have no problem holding it open and turning pages.
3) You can carry your entire library with you. Great for trips.
4) No more gross "who knows what that is" stuck to the pages when reading library and used books.
5) Automatically saves your place in the book.
6) Highlighting doesn't cause permanent marks. (I highlight a lot so this is a big one for me)
6) Built in dictionary.
7) No more cracked spines and losing pages from old books.
8) Tons of free books available.