I can't tell you about the Kobo device, since I don't own one, but I can say that the Kobo books have been great. I can download directly to my Itouch via wireless. I've been able to sideload them easily onto my Nook using Adobe Digital Additions. There has only been one title that was problematic reading into Calibre. They took a return on a book I accidentally bought at another place.
I've read (here? Somewhere else? Don't remember) that potentially, I could run into issues buying a book that isn't licensed in the US paying via credit card. I pay via Paypal - I buy a gift card every so often for $30-50 and use that to make my purchases. No problems yet (82 books).