Got my 950 yesterday and have been playing... Thought a thread comparing and contrasting to other readers we've used and liked would be interesting.
Kindle 3's pearl finally made me soften my death grip on the 505 and upgrade... Was very pleased with the K3's screen and speed... The firmware, I find somewhat goofy. Didn't like the PDF support, but eventually found a barely acceptable method for reading my technical PDFs using PDF2LRF and Mangle. The main downside being no Go To navigation using the picture viewer... Keyboard I never used and found it rather large and awkward. WIFI used once, to check email from bed, being super lazy.
Never even registered it with Amazon.
My initial thoughts on the 950... Looking at it right next to the K3 it appears to have less contrast. At least part of this I attribute to the figure-ground effect of the Kindle's blackness vs Sony's much lighter silver. But also the Sony's customizable contrast cannot match Kindle's contrast on some documents. I think this is mostly due to Sonys fonts appearing less substantial and more pixelated than Kindles... I think the gap between Sony's small and medium is too large a jump.
IMO Kindle's fonts are quite a bit better. Hopefully workarounds can fix this. At first I thought Kindle's default font was cutesy, but now I miss it. It really is a very readable and comfortable font. I'll have to put it into some of epubs and pdfs to see how it looks on the 950...
Page turns on the Kindle are significantly faster than the 950. A little disappointing, hopefully firmware can fix this (ie, like one of the earlier Kindle 3 updates did). 950 seems to pause, then page turn. Hopefully this pause can be fixed because otherwise the page turn seems equal to the Kindle.
The aspect of the 950 does very well on cropped, regular sized PDFs. This factor alone will make me migrate from the Kindle... FINALLY, I can read PDFs, have access to search and dictionary without having them converted, rasterized and/or broken (or glare screen). Touch screen is a plus mostly because I dislike Kindle's mega keyboard and it will be nice to be able to take QUICK (not like Kindle) notes again.
All in all, while I'm a little disappointed in the 950 for not being the undisputed superchamp I was hoping for, I still think it is the best reader on the market.
Another factor... I don't ever see Amazon going out of its way for fringe user like me. On the other hand, a big future factor for Sony is PRS+ the alternate firmware. Greatly extended the function on the 505. The author was just donated a 650 and 950, so here's to hoping for great things!