If you buy a book from Amazon you cannot swap it with your friends. Bought books have DRM (Digital Rights Management) which prevents you from swapping them with a friend.
Amazon do allow you to 'lend' a book to a friend although while you do this you cannot access the book yourself. Why anyone would want to use this feature is beyond me, but never mind.
The kindle solves the problem of blurred vision when having the eye drops as you can make the text bigger. I thought this was a fantastic feature as it always used to irritate me when waiting I couldn't read my book. Now I can
I think once you get used to a kindle, you'd never go back as its pros far outweigh any cons.
The only con I'd say for the kindle is if you want a reference type book. One that you need to flick from chapter to chapter or looking up something in an index. Although it is possible with e-readers, it's far quicker and easier with a traditional book.