Hey, I did too.
Just played three rounds of coop, one was laggy (well, slowed down) but afterwards one player dropped and the following ones weren't laggy so I suppose that was the fault of that player. I wonder what equipment the other guys were carrying, my lv3 SMG (better than any "archievement unlocked" for beating the hard difficulty) certainly served me well. It didn't look like the other players knew where to shoot a beetle (I call 'em elephants) when attacking from the front (hint: between the legs).