Certainly a game worth buying, whether you have live or not.
Aiming works great. You are able to customize your character in the single player to a satisfactory extent, and upgrading skills makes a HUGE difference, which makes upgrading fun. Lots of gadgets and gizmos to buy and equip yourself and/or team with.
The enemy A.I. is quite good IMO. It's truly better than most a.i. I've played against in other fps, and I've played almost all of them.
The multiplayer is really where this game shines, on and offline. Offline, you can do four-player tournament co-op, or just play exhibition matches against bots, both are a lot of fun. Online is great. I haven't played it too much, but the time I did spend online was well spent. Only problem is that there aren't a whole lot of people.
Graphics are good. Not Halo 2, Doom 3, or half-life 2 quality, but they are more than adequate.
All in all, this game is just plain fun to play. The short, intense rounds make for a very entertaining gaming experience that I expect I'll be coming back to again and again in the future.