I've just bought a 505 (2 days ago I'd never heard of an eReader. it was an advert for the 300/600 that piqued my interest - I decided on the 505 for most reasons mentioned in this thread), lovely device in general (read 2 books in the first 24 hours!), I just wish it had 2 big fat buttons (each side) for page turning rather than the diddy ones on the device, they're far too fiddly IMO.
The library service is excellent (
http://essex.bookaisle.com/ ), the cost of the device will be recouped very quickly from the amount of books I'll borrow.