I have a Sony 500 and a 505 (no contest, as we all know - 505 wins, hands down) and bought a K1 last summer. I was expecting to use it for the instant books, but then use my 505 for all the other reading I do. I'd set up a work flow in Word that allowed me to take a file and convert it to a Sony-sized PDF, so it was fairly easy to get content on it.
Wow, was I surprised! About a month ago I actually had to dig out the 505 because I was looking for a specific book I was pretty sure I'd put on it. I had not used it in so long, the battery was completely dead. Had to recharge it before I could locate the book.
I realize it is all personal choice, so I'm not bashing the 505 at all - it's a great ereader. It just surprised me that I had taken to the K1 so readily and so fast that I hadn't done anything to 'put away' the 505. Just left it where I'd last used it before getting the Kindle.
The biggest plus for the Kindle for me is that I usually use Macs and since most of the interaction on the K is OS-independent, it's quick to put content on it even if I don't use the email service (which I often do).
The Sony, on the other hand, required that I boot up my Vista laptop
and do all the work I mentioned above. I did buy a few books from the Sony store, but not many and it certainly wasn't the experience that buying from Amazon is!
I didn't move up to a K2 because the K1 was (and is) fine for what I need. I have since added the DX for reading PDFs (full-sized ones) and I'm loving it. I'm a bit more diligent in keeping the 505 charged up - just in case - but at the moment, that's all I'm doing with it. (I very much miss that great overlay light that Sony has, though. The Kindles would be even more useful with that.)
In other words, both the 505 and the Kindle are great ereaders (imo) and it's more the experience of it rather than tech specs that determine which one is right for any given person.