I have one. I think it's very good, however it really is most useful when you have jailbroken your device and can use Blutrol. Otherwise you're a bit limited. Yes, it can play all iCade enabled apps, BUT many of them have bizarre button mappings on the iControlpad which renders them almost unplayable. For example, Muffin Knight puts the attack button to L shoulder on iControlpad.
I had an iCade as well, but sold it recently due to non-use. I prefer a small gamepad style device that works well with my Touch as well as iPad.
If only Apple would stop being so stubborn about gaming. Give us an official physical game control interface.