I have had a Sony PRS505 reader for nearly two years now. I like the ability to take lots of books with me when I'm away from home and that alone makes it worth the money. Apart from the 100 or so 'free' books that come with it, all of which are 'classics' or to put in another way out of copyright, I've bought several books from Waterstone's, Amazon and W H Smith as well as getting a number of free books from various free ebook sites.
Currently it has some 80 books on it and I'm going on holiday soon so will be able to take them all in a 'slim volume'. In the past I've taken three paper books for a week away and still run out of reading material.
Pluses, as above, easy to read, you have three print size options, battery easily lasts a fortnight away from home so no charger needed, the unit will work with almost all formats (except Microsoft reader!)
Minuses, price, I paid £200 for the reader and ebooks costs are frankly not a lot cheaper than discounted paperback prices and can be as much as a hardback for new releases, (This is a rip off give production costs are minuscule compared to paper and as the market grows surely competition will drive this down.), purchase sites are almost all hard to navigate. Some will search for eBooks and the results are a mix of e Readers, eBooks and paper books, others have multiple formats some of which are Audiobooks in the list. The market is young ans still evolving, the first time I shopped for a new ebook was a nightmare, it's still not easy but W H Smiths has the best site so far, although it's still not that easy to navigate.
Big plus. My wife no longer rolls her eyes when I tell her I've bought yet another book, our living room already looks more like a library than it should.