Yeah, like I said, it's super cheap (not free, though -- lol), complex, and RUTHLESS. The main difference that I should warn you about is that in the single player missions, you're contolling the entire team. You set up a mission with waypoints and attach commands. So for example, when you hit the key that gives the command "Alpha -- go!", the guys you've designated as your advance team will run to the first waypoint and do what you directed (wait, snipe, smoke and engage, etc). You can switch to any guy on the team and play the way you're used to at any point as well.
It takes some getting used to, and it's brutal, but really cool. When you start playing online it's more of the traditional Counter-Strike / America's Army style, only it seems like the player community is a little more mature because the single player game is so tough and complex that it weeds out the kids and the morons.
