I was very skeptical about the game becoming a run n' gun twitchfest with the inclusion of fast zombies, and, it kind of is, but it's deeper than that.
I'm extremely satisfied with the game so far. As long as you get some non-assholes to play with, it can be very tactical. It's not Rainbow Six, but you have to be aware of your surroundings all the time, stick with your team, watch your shots so you don't HIT your team, manage your ammo, etc.
The Director does a great job at providing those "oh shit!" moments, and the levels have some decent variety to them. TF2 twitchfest this is not. The finale on No Mercy was CRAZY. If you don't work together on the upper difficulties you WILL die and die again. And respawn can take a while, over a minute sometimes. There's plenty of tension and incentive not to screw around.
So far I like Advanced, it does a good job of pacing hordes and special enemies. Expert, the little I've played it, seems to throw TOO much at you. But perhaps when I get my skills up a bit and know the maps, it'll provide a challenge.