I have one of these I bought for my Nexus 4. It worked horribly since the back of the Nexus 4 is glass, and would slide off the angled charger after a few minutes. I completely stopped using it after a couple of weeks of trying to get it to work well.
However, I got a Nexus 5 recently, and since it doesn't have the glass back, I decided to test out the wireless charger again. It works much better, although I really wish Google would allow people to trade in their old wireless chargers for the new design.