I owned 3-4 MP3 players before the iPhone, and the iPhone is the one I've used the most so far. The interface is easy, my gripes are pretty low. The second best MP3 player I owned was an Insignia DV-4.
I learned that there are plenty of MP3 players out there with more features, but the usability is the key, and the iPod UI itself is pretty good, which annoys me, because I didn't want to like it.
They aren't this popular because of a fluke, it's actually a good player, and that pisses me off.