I have always been a fan of the Call of Duty series. I just got finished playing the CoD4 demo and it was the worst demo i have ever player. . . because it was the shortest demo i have ever played!
My current favorite game is Battlefield 2, but CoD4 blows it outta the water. The demo was fantastically amazing! its a lot more realistic than other shooters including BF2 from the way you reload the weapon in different states (whether or not their is a round in the chamber) to being able to tag enemy combatants from behind concealment. even the night vision was much better than other games and features the infrared laser sight; a sight that can be seen with nvg's but not with the naked eye. i can go on forever. i need to get this game. halo? pssh.
amazing game, highly recommended!
http://callofduty.com