After playing the beta extensively, I've come to a few conclusions. Some of you have either played very little of the beta or weren't paying attention while playing, since I have seen impressions that are simply false. Even from some that I usually considered to be reliable posters. Especially regarding co-op.
Regarding co-op enemies, when you get to higher levels, their agression increases dramatically and they become much smarter. At first running and hiding would work for me, but not anymore. I saw a very impressive attack from the Chimera yesterday where they were literally seeking out and destroying every living person on the map. I know because when they finished me off, they immediately turned their attention to one of the few who had lived through their initial onslaught despite the fact he was not shooting at them. They also were evasive and used cover to protect themselves from me before rushing where I was holed up.