Well I'm not that deep into it yet, but so far its passed the initial "is it fun to play". Whether or not the game play holds up, we'll see.
Crackdown didn't have a ton of variety it its missions, goals, or combat but was still a ton of fun for me.
So far Prototype has a lot more options for how you fight, different powers and weapons. And the storyline is pretty interesting if nothing special. Government black ops, mysterious virus that was likely engineered, blank protagonist, etc. But the atmosphere and game has been a lot of fun. In particular I like that you get a lot of the backstory in short bursts after you absorb the people whom have the memories that fill in what is going on.
I like Xbox's line up overall. Its a bit of a lull at the moment, but I need to catch up on games anyways. I'll probably be picking up some used or maybe reactivity my gamefly account until I start school again.