I have a PRS 505. I haven't played around with any other ebook readers. My husband did a lot of research and decided 505 would be best for my needs (it was a birthday gift).
What I love:
It's easy to use-- to me it feels just like holding a book.

It's super easy to read, especially in bright light. It fits into my medium sized purse and can go anywhere I go. The cover fits great. I love how easy it is to use, how I can get books in a variety of formats. I am going through the ebooks at my public library like wildfire and it's so easy to do on the sony. I don't like to pay for books, so I love that I can easily get the 100 free classics from sony (I know there are other free books out there I can get, but I'm not ready to delve into that realm yet... baby steps for me!

) I love how easy it is to organize the books into collections. Although it's an extra step, I really like downloading it to the computer first. My husband accidentally deleted 2/3 of my collection from my ebook reader. It was super easy to re-download because it was either on the computer (saved) or in the sony store.
What I wish I had:
a dictionary where I could easily look up words-- that, I think, is the main advantage to the kindle, I wish I had a light, but I don't like backlit, so what I really want is to just get a booklight to have next to my bed (so I'm not really expecting my ebook reader to have that feature)
Honestly, I'm glad the choice was *not* mine. I was initially a little disappointed I didn't get a kindle from my husband. I had really only heard about that, and so that's what I thought I wanted. Now, after 2 months of using this and doing "after the fact" reading about both devices, I'm really really happy with my sony.