This game is awesome in multiplayer - my personal favourite is Terrorist Hunt, which sees some strategic moves being made by your group in order to eliminate 30-40 terrorists from an area. On realistic difficulty, a few shots will kill you, so caution is the name of the game. This is what makes the game so fun - the constant fear of death, but you gain a great deal of satisfaction from completing each level. It's also great for 2v2s; personally, I've played with rotating teams and had a blast with every person, good or bad (skill-wise).
The story mode is also fun but rather short and unoriginal, but it can be played with a friend online or off, which makes it awesome for the same reason that T-hunt is - you need to communicate with your team mate. Telling your friend your position, the position of the terrorists, sightings, possible entries into buildings and other information is crucial and can be hard to do well, but with dedication comes a game where you feel like you're part of a skilled team.
The EXP system is another high point of the game. Every single kill, whether online or off, counts towards your overall rank. It's very well implemented as it feels like you're making progress in the game no matter what mode you play.
It's not a game for the faint of heart - I know people who have returned this because they immediately made the assumption that every shooter is Halo or Gears-like to play, only to get their ass kicked when they ran into their first terrorist. It can be a waiting game or it can have battles of terrorists killing everything with explosions going off everywhere, but at all times it remains a tactical shooter.
One of the most complex console games available on the current generation of systems, and one of the best games to have a team playing. At a tenner, it's a bargain.