I played zombie master, and was pretty much instantly repulsed by it. The standard there doesn't seem to be high-quality maps. More so really, really, REALLY awful maps. Seriously, one I played on was just 'know where the things fall that kill you', another was 'move from room to room and shoot at zombies that slowly float at you with near-infinite ammo', where rooms were literally just chunks of brush suspended in the skybox, and the third must have been built by a second grader who LOVED neon colors.
If it had good maps? Maybe. I don't think *slow* zombies would work to well in an actual objective-based game (which it is), though. If the zombies can't get to you, then they just become a slowmoving area-denial tool. Actually, that could be an interesting mechanic if done correctly, but more likely than not it wouldn't be the kind of game I'd want to play.
All in all, the problem I've witnessed there is level design. I'm sure good levels exist, but I haven't seen them.