I'm eagerly expecting e-books become more general as a publishing form, so I bought kobo, released by Rakuten in Japan, on its release day and thought it was not so bad than said. I think Mikitani-san didn't well to subdue the bad reputations about kobo, though.
Personally, what the device offers isn't so much important, but it's eco-system that matters. I've been using Amazon for 10 years or so. I rely on their service; I expect them to release good books and save them there safely for long time with sustainable business models. Thus, I'll use Kindle as I've been.
The e-book market in Japan is so poor even now when kobo finally entered it. There's almost no difference kobo's market and Amazon's as far as Japanese books. It just only started in these days. Kindle is reportedly going to be released in September. I'm looking forward to that they change the Japanese book market drastically.