Definitely worth the money.
The single player campaign of Far Cry is definitely the best I have ever seen even considering Halo. The game is told entirely in first person, including the cut scenes. There is little difference from the cutscenes to the actual gameplay aside from the whole, ur not controlling anything. I think that one thing that would have made the single player perfect is if they implimented the whole Resident Evil 4 system where you would have to press buttons during the scenes to save ur character. It keeps you on your toes.
The game starts out with you on your boat near an island with a girl talking to you about borrowing a seadoo attached to your boat. She soon leaves and then you are attacked by unidentified soldiers. Instantly you are thrown into a battle for survival armed only with a knife. You get tons of plot twists and awesome weapons throughout the game. One thing I would complain about is the voice volume. The music is almost always playing and sometimes it's incredibly hard to hear what they are saying. And since most of the story is told in battle, it's hard to concentrate on whats going on in the story. It's incredibly annoying not knowing where you are supposed to go. Another cool thing they added was the ability system. Certain events happen during the game that lead to you obtaining animalistic abilities that are no less than super-human. These include running and jumping extremely fast and far. Also you can "see" scent trails of enemies. While the later isn't extremely useful, its kinda cool when your stalking an enemy or if ur in a dark area.
On to the multiplayer. This is, in my opinion, one of the worse parts of the game. Halo is currently the FPS standard for games and Far Cry's just isn't interesting enough to cut it. I haven't tried the xbox live yet but the offline multiplayer just isn't that great. Every once in a while, you get an interesting battle but generally the battles are quick and not altogether exciting like the halo battles. Since there arent that many weapons and most of them do the same thing, not that cool. Aside from the sniper and rocket launcher, the weapons are grouped into a pistol and rifles. You can carry one of each and a special weapon (sniper/rocket launcher/chain gun) that arent all that special.
Overall, if you want an excelent single player FPS that compares with Halo, this game is perfect. If you are looking for a multiplayer game that you and a bunch of friends can play and stay hooked for a while. Don't bother with this.