I never played the first but a friend and I got hooked on the co-op beta for the second. It was actually quite fun when playing with a bunch of people who had no idea what they were doing. I remember one instance where I was a Spec Ops all by myself, fighting backs dozens of the charging zombie dudes (grims, I presume) and the flying robot things for ages and right before I would've died my friend shows up as a Medic and makes everything alright. There were quite a few times where we were left keeping the team going.
So, yeah, we wound up playing co-op in the full game a lot. I had gotten really good at Spec Ops while my friend was a prolific Medic (we usually fought for top spot on the leaderboard). I think the mode worked really well in terms of the classes, scoring and overall design.
I tried the campaign but it just didn't interest me. It started off pretty dire in that underwater base or whatever and even included some annoying pseudo-platforming. The bit in the woods was pretty atmospheric (the cloaked dudes were dumb though) and I think I just got bored with it shortly after that.
I haven't really read anything about the third but if it has a similar co-op mode I may grab it at some point.