I love my Sony PRS650. I buy most of my books through the kobo store and its easy to transfer over to my reader. I'll eventually get a Kindle 3( because I am not into DRM stripping) but right now I'm 100% happy with my 650.
Kobo.com is also compatible with quite a few non kobo readers
http://www.kobobooks.com/ade
I haven't bought anything through the Sony ebook store so I can't comment on that but so far I have found what i'm looking for at a lot of other sources.
My only hesitation about buying a kindle3 is not being able to see it before i buy it plus the shipping there and back if i have to return it. Though I'm sure curiosity will win out lol