I've got to say that it's worth a rental and nothing more. In fact, I'd say that for killer7 as well.
I'm a big Nintendo fan and I gave Giest a chance. It starts off okay, but honestly, the levels are poorly designed and the action is bland and boring. The "puzzle" solving is a huge dissappointment too, as, in general, you don't have to actually solve anything. You just possess something and push A to move things along. This game had a bunch of potential, and it was in development for a relatively long amount of time. I would say it was a big dissappointment except that you could kind of start to see what it was going to be like as the released date closed in. Being a big Nintendo fan, I got it anyways, and yeah, I just wouldn't recommend it. There are a lot better games you could spend $20 on. Also, if you really need a GameCube game, ChibiRobo came out today and it's $40 at Fry's. It's a much better buy.