I just tryed a pair of them on a 300 watt heater with mixed results... They seem to be holding the heater just fine but on the other hand I don't think I'd ever go out of my way to buy any more. (these are samples)
They do make adjusting things easy but they can also get knocked off so it evens out in the end.
The only one I have used came with the powerhead as the sole source of attachment. It works very well. I like it a lot better than suction cups. All my suction cups end up at the bottom of the tank attached to nothing.