I haven't played with WP7, but I still claim that the Zune HD had by far the best interface of any portable MP3 player. I sold mine recently, and I really miss the UI elements that thing had. I don't know how much of that carried over to WP7, but it looks like quite a bit to me.

However, there were constricting elements to the way interface was designed in everything but the music player, so I'd probably be hesitant to try WP7 based on that. Besides, I need a real GMail client on any phone I use...