I'm still not enjoying Exterminatus' idea as much as Synapse's idea.
I mean, all those ideas are pretty good but... why fight some mobs when you can fight players?
It's 1000 times more fun, like playing UT2004 with a bunch of friends and with the bots. It's another story...
If I want to kill "bots" I play single player games. I still think that Exterminatus is, by force of things, going to be boring, since:
1)bots are bots. They can be intelligent as hell (just look F.E.A.R.'s enemies) but they aren't real players.
2)I don't see much competion in "hey this time I'll resist more than the previous one" or "more than you". I mean, it can be fun once, twice, thrice, at the 4th time it's boring.
3)I don't think that the devs could do all those great works in a short time. This would lead to playing 3-4 missions for some time, it gets boring, then the devs put out a new mission, everyone plays that, and then it gets boring until a new mission.
4)You are ALWAYS a Space Marine. I mean, they're my favourites, but sometimes it's just cool using some other race. Being able to switch playing styles add lenght to the games' fun.
Those are my opinions, and don't worry, I'm not killing who thinks something else

I just think that Exterminatus looks good, but Synapse looks better (and we can't never say: maybe in the future we'll have both playing styles available).