I've used (and do use) both the G3 and the Bilt Hamber clay bar. The mitt is easy to use, but you do need a quick detailer to use as a lubricant. If you get a clay streak on the paint, it's a real pain to get off again.. I have to use a scratch remover or similar. It doesn't take much, even a dry spot can lead to a deposit.
I think the Bilt Hamber is the easiest and most effective clay bar I've used - really gets tar spots and bug splats shifted almost immediately, and just being able to use water as a lubricant is a clincher. It's also cheap for the quantity and lasts very well.