It may depend on the iteration you had. I remember very clearly that the touch interface was pretty good for a resistive touch screen, but I found icons and the interface to be made for ants and that was back when I had better than 20/20 vision. I also do remember that apps were pretty sparse.

I think the interface was better than Palms interface and Blackberry's but the iPhone in particular had a better easier to navigate UI, and android (even though they took a little bit to catch up), also generally had a better interface.

I feel like if you were a windows user it was fine. Other than that it just wasn't that great. But they didn't even try to compete once they got actual competition.

Edit: My experience with Windows phone was from the Windows phone 5 interface (back when it was similar to the desktop client before they did their tile touch iteration to make it more like windows 8).

So in fairness, it did get a little better but that was pretty much right before MS gave up on it entirely.