Green Onion aliens with asses on the back of their heads? Sure. Why not?
I'm on about the sixth episode of GANTZ right now and I have to say: this anime does not disappoint. It is gory. It has mutants. It makes little logical sense. So far so good. That's not to say that it doesn't have its annoying points. For example: I don't really need to listen to the whiney inner-thoughts of the protagonist. It was okay for the first few minutes, but what the hell? And why did the whole "secret government agency" element even have to be introduced? Being stuck in Purgatory wasn't a strong enough plot? The animation is great, though. And the episodes fly by. So far, I'd say this is an above-average anime.