Most problems with KZ2 seem to be related to your NPC partner.
It wasn't a crash, but I had one instance where I had to backtrack to pick up my partner who was stuck behind an opened door while trying to run through it.
The only game stopping bug I encountered was at the very end where my partner managed to get himself incapacitated INSIDE a pillar as though he teleported into solid matter.
I didn't see how it happened, but I think an RPG explosion pushed him into the pillar. Game wouldn't let me revive him as he floundered about inside the pillar shouting angry lines and the game couldn't progress until he was revived. Ended up having to die to restart the area.
Easily the weirdest glitch I've seen since Fallout 3 with the flying, twirling, dead super mutants. I should have taken a screen shot.
As far as slowdowns or run issues; none on a 40GB model. They're just not there. No freezes, restarts, severe frame drops, etc. The only hickups seem to be the brief moment in between area load times when there is no action and the "loading" indicator shows.
Observations are based on a bit less than a 7 hour run through on campaign, plus a few hours into a second run through, plus an hour or so of warzone play.
Far from broken (unlike Fallout 3 on PC), but it is odd that a game that has had this much development time for a console would have any serious glitches at all.
Hasn't changed the positive playing experience yet.
I'll probably play it on the 60GB model next to see if there's any difference.