I play Ghost Recon quite a bit, and I thought the demo did a good job of showing what the final game is all about (but then again I've also had Rainbow Six and Rogue Spear before Ghost Recon).
Within the demo you are given three options on a single mission map. The "mission" has objectives that need to be completed to finish the mission. In "firefight" you have to kill all the enemy. And in "recon" you need to get from the entry point to the extraction point alive. The game is not a standard first person shooter. One shot can (and usually does) kill. When the AI characters see you they know to run for cover. They can be very good shots. It is very much like a virtual paint ball game. There is no "health" to help you out. The ammo that you have is all that you get. You have to learn to be careful and sneaky.
In the full game, the people on your team get better with each mission. After a few missions some of your guys can become very valuable which makes you less likely to do something stupid with them (I usually send new recruits into harms way before veterans). The mission objectives change. Sometime you are securing an area, somethimes blowing up a target, and other times rescuing people. The full game comes with about 35 missions. Once a mission has been completed it's map is added to the list of quick missions you can try (where like the demo version, you can do either missions, firefight or recon with each map).
I like it, but then again I enjoy the fact that it is not like the standard first person shooter (like Doom, Quake and Unreal). It really feels, like I said before, more like the experience of playing paint ball with a group of real people.
It is not for everyone. My wife doesn't like it, she likes playing Unreal Tournament more. If all you want to do is blast people, there are better games for that. If you're looking for something more then running into a room with guns blasing, this is a great game.