I have owned a Sony PRS-500 for over a year, so these impressions are heavily influenced by my experience with that device.
The Kindle is a little larger than the Sony, but it's not an issue for me. It is very easy to inadvertently turn the page when passing the device around to co-workers (or flipping the device over), but I haven't found this to be a problem when just holding the device to read. It is at a slight downward angle. I have no idea why Amazon did this, but I didn't notice it at first. When laying flat on a table, the device is thicker on the left than on the right. I haven't found this to be a problem or to be annoying when reading either -- just strange.
This device shines compared to the Sony in many ways. It is very easy to place a note or highlight a section within a book, and then browse through those areas later. Looking up definitions in a line is also very slick. Being a Mac user, I always had to purchase and sync books on the Sony through a virtual machine. I love the fact that I can do all this wirelessly from the device without a computer at all. If only J.K. Rowling and Robert Jordan's works were available, I'd be in heaven.
The cover it comes with leaves a lot to be desired. It's not nearly as nice as the Sony Reader's integrated one. The Kindle's seems to be cardboard surrounded by fake leather with an elastic band to keep the case closed. The jury is out on whether this will be enough to prevent the device from falling out of the case in a bag or backpack.
Overall, I'm impressed. It is too bad that Amazon couldn't open up their DRM to work with other devices, but since I was already locked to Sony's DRM, this is just more of the same. Except, now I don't have to plug into a computer ever.