I've had two, and it is a great idea executed poorly. After about a month, the gears start to slip and it gets stuck into one position. You then have to take it out, clean it, put it all back together and then it will work for about another day. I took them both off because they drove me nuts.
The problem is that there is a plastic shaft that holds the rotating part on the powerhead that is a little too long, which means that when the water is pushing on it the gears don't make contact. It seems like an easy fix for Hydor, but as far as I am aware they haven't fixed it yet.