Haven't had it long, but I figured I'd help those on the fence about it with whether to pick it up or now.
Did I mention there are a LOT of guns? K, there's a LOT of guns, allowing you to fine tune your gun to your personality. I've grown attached to a certain handgun, assault rifle, and shotgun, personally. As for the missions, I can't say they're different from what you've done before, but I can this game takes what has been done and does it incredibly well. Mark my words, you'll want to keep playing once you start it, and you will have fun, you will laugh, and you will want to experience it with some friends. The split screen works well, and it, also, is really fun.
From what I've played so far, I'm 99% sure that I want to keep playing til the sun comes up; but lo and behold, college waits for no one...