The mission path is fairly linear /but/ you have a lot of places to explore and you don't have to take one path throughout most of the game as you can take side routes to get to the objective and you may discover items of use on the way. The best part is the dialogue and characters though, they're all pretty personable and have their own little things they do.
The only thing that lets it down at times for me is that the enemy soldier AI is a bit naff at times whereas the infected AI is very good. I threw a grenade in front of a guy and his mate didn't even hear it and would only come fight me once I was within his line of sight...
Stick with it, 2 hours is still early days