I also had the Iphone3 version of this charger and it worked every time. The Iphone 4 version unfortunately is quiet fiddly. I find myself spending a few minutes tweaking its position to find any charging point on the mat, it can get annoying. Also not sure why you have to snap the charger closed am worried it may scratch the iphone, there was no snapping mechanism on the Iphone 3 version.
When it works it's good but I'd recommend trying to find the Apple Tip for the powercube charger, which appears to be only for sale in the US.
It does what it says on the tin (hence 3 stars) if you can get it working but I'm a bit disappointed with this.