I bought a Kobo because...
a) The Kindle was a bit too restricting and Amazon is playing the game a bit sideways, which I don't like. (they lose money on the hardware, but make it by locking you into buying pretty much everything from them)
b) The Sonys were powerful, but outdated and seemed overpriced. If the new models had come a few months earlier it would have been a much harder decision.
c) The Kobo was immediately available at my nearest bookstore/Walmart, the price was right (got in on the first $128 weekend) and by all accounts it was a good solid eReader and nothing more, which is what I wanted.
d) The Kobo group, seem like a progressive thinking, risk-taking bunch. It seems like they are treating the Kobo as a platform device which means it may end up as a much better product than it was the day it first hit store shelves. The next firmware will apparently add two of the most requested features...that says something...I can't see someone like Sony doing anything more than 'fixing' bugs with their eReader firmware.
This was evident to me in my initial research using sites like this one.