Serious Sam 3 launched recently on Xbox LIVE. If you've not picked the game up yet (and I recommend you do), perhaps today's trailer will convince you. Featuring about two straight minutes of Sam pumping various beasts full of lead, rockets, and cannonballs (cause why not?), it does well to represent the game for what it is: an arcade-style shooter with near endless action.
Remember the Xbox LIVE version of the game unfortunately had to be split up into two parts: the single player and co-op portion, and the multiplayer and Jewel of the Nile campaign DLC portion -- each is 1200 MS Points.