Definitely, the game I've played most on XBox Live. Forget Halo, this has it all.
Some let downs would be the AI. That being the multiplayer and the story. The story is all fine on the hardest difficulty until the last couple of levels. The "bosses" aren't too god either, the have little or no weak spots, so you can just spray into them, regardless of their attacks. You also get a companion in most story levels, but they go off on their own missions, or get into traps. But again, they aren't too smart. If they get fired at, they flinch, but granted, you wouldn't just stay still. They flinch if they get fired at by a rocket, which really is taking the "Flinch system" too far.
On the other hand, you can customise it to ever extend. Rather than playing Slayer on Halo as a quick match, with default settings, you can make it to your personal preferences, making your multiplayer even mre impressive.]
Now onto the graphics, the graphics aren't exactly amazing when you compare them to something like HL2, which was released a year previous, but they have improved vastly since TimeSplitters II.
So, in conclusion, this is definitely the game I'd choose when it would come to run-and-gun, fast paced first-person-shooter action. Especially when there's Undead Priest still there to use ;)