It's quite simply a blast to play. I still can't customize things yet but playing online with random folks is fun. You can play single player too, don't get fooled by the co-op that they slapped on the product name. But it's mostly to get a feel of the controls. It does take a little getting used to, but once I knew how things work, it's not a problem.
I think the demo is only on the US/Japan PSN?
In short, it's like Monster Hunter, with guns. And that's a good thing.