A few more additions and answers to questions:
The scroll wheel is really nice. It's very quick to select a menu item. In contrast, on the Sony reader, you have to wait for a screen refresh for every item you scroll onto. The LCD (the technology name escape me) the wheel controls has a neat feature in that it doubles as a progress meter. It will rotate or expand depending on what you are doing/waiting on.
I haven't had an opportunity to try to convert any files PDF or otherwise. That's on my list, but I haven't had time yet. You first need to setup the e-mail address through amazon.com according to the manual, and you also have to configure which e-mail address it can receive from -- so not anyone can mail your kindle.
As for speed and contrast - it definitely feels faster than the Sony, and side-by-side it turns pages faster. How much faster? Tough to say - .25 seconds? .5 second? I imagine the speed difference is comparable to the Sony 500 to the 505. Contrast is also comparable. It's tough to say if the screen is lighter than the 500 or if that's just the white plastic around it making it look that way.
When you hook the Kindle up to a Mac via USB, a message is displayed telling you that you can't use your kindle to read and the wireless is turned off. It is mounted just like a flash device.
I agree with sfernald's statement - aside from finishing a book or two on the PRS-500, I have no reason to go back to it.