I purchased the Sony eReader right before I deployed to Iraq and it has been great. I am working with folks that have iPads, Nooks, Kindles, and the Sony and we have been comparing notes.
In my completely non-scientific pole, I have discovered a couple of things:
iPad - This is the only device of the ones above that is "backlit", so it can be read in relatively low light. All the others need some sort of external light source. It is also bigger, more expensive, and has a shorter battery life than the others. BUT, it can run a lot of really cool applications that make it "kind of like" a regular computer AND there are apps from all the other eReader manufacturers that allow their ebooks to be read on the iPad. Since we don't have 3G or wireless LAN capability where I am, the iPad folks have been very frustrated trying to download books. I was able to buy a new laptop AND the Sony eReader for the price of the 3G iPad before I deployed and I'm glad that I went that route. Maybe I'll get an iPad next year....
Kindle - I think that the Kindle has the best e-ink of the lot. I think that there is a little less eye strain and glare on the Kindle screen than there is on the Nook, iPad or Sony eReader. The Kindle also has a little bit bigger library than Sony or Barnes and Noble. That being said, folks with the Kindle over here have had a hard time downloading books from the website to their computer, then syncing them to the Kindle. They had no problems when there was 3G availability in the US.
Sony eReader - I really like my Sony eReader. As I mentioned, the screen isn't quite as nice as the Kindle, but it is still very readable and doesn't cause me any eye strain, especially after I swallowed my pride and enlarged the font a little bit. This is the only eReader that the entire reading screen is a touchscreen. This is not a function at all on the Kindle and the touch screen portion of the Nook is really small at the bottom of the device. This allows you to "sticky note" books, draw pictures, etc right on the device, which is a nice feature. The Sony also has the least amount of propriatary restrictions on the types of formats that you can import to the device. I have pdf files, tagged pdf files, library books, jpeg pictures (black and white only), and audio books all on the device. It also has the easiest expansion capability with SD and PRO duo chip slots right on the side (not that I ever think that I will need the extra memory). I think that the main selling points for me were that it had an aluminum case (nice for my current location), came with an external 120/240v charger (in addition to USB charger), and that it allowed for a broader range of media (pdf files, etc.) to be imported. I have had no problem downloading purchased or library books to my computer then transferring them to my eReader (at home I just used the 3G).
As far as price per book goes, they are all about the same. Most books that have been out a while sell for about $7.99 and new releases are in the $12.99 range. In all cases, it the books are cheaper than buying a hardback book. All the manufacturers have some provision for the classics to be free or only cost $0.99 or so.
I wasn't sure if I would like an eReader since I am used to holding a book in my hand, but I needed something that had a long battery life and didn't take up a lot of space for the entertainment value. I am now a huge convert and my wife is getting one for Christmas. I have read about 25 eBooks in the past 4 months and will have a really hard time going back to paper.