As any electronic device, the Kindle takes time to learn.
In addition, it has its quirks. For example, sometimes books appear to disappear
, when in fact they for some reason go back in the queue. Once, I start the wireless connection, the Kindle syncs with the server and everything returns to normal.
I've had a Sony e-reader and I've read the reports about Nook. So far, my opinion is that Amazon's Kindle is the smoothest experience. I love it.