Kathryn, I haven't even used mine yet, but I can tell you that you can definitely adjust the size of the print. That is a very attractive feature for many. And my understanding is that the book is yours forever on your Amazon account, whether or not you keep it on your Kindle. I think it's like the itunes library. Your purchases are always there, regardless of what songs you are carrying around on your ipod. Those in the know will have more to say about this.
My primary reason for buying the K2 is that I live in an apartment, and I just don't have room to store hundreds of books. Probably even more important, I don't want to! I despise clutter, and I despise paper clutter most of all. Obviously, there are some books that I keep, like reference books, etc. But when I'm finished reading a novel, I never go back to it again. And I just don't need it hanging around my living space. Also, I commute on the subway, and the K2 is light to carry around. Oh, and if you travel, even if it's just once or twice per year, you can carry all your reading material on one little featherweight device. So, you see, you NEED ONE and so does Michele. (evil enabling grin).
ETA: The biggest drawback to buying a K2 is the initial layout. It costs $359 and doesn't come with a case, so you have to add on at least another $30 or $40 for a case. However, there are no subscription fees or hidden costs. You just pay per book or periodical, and so far the prices have been good for those. Of course, Amazon does have the right to change its prices if it wants to, but many K2 owners are saying they will boycott any book over $9.99, including new release hardcovers.
Sorry for the novel.