Every once in a while it freezes. Usually on a loading screen. If the letters are still flashing, it's not actually locked. Sometimes it just takes a LONG time to load. However, if the letters aren't flashing, it's locked.
Sometimes Niko loses his hat. I believe it's when he gets into a car. Sometimes the hat returns. Other times you have to go home and put the hat back on. I paid $8 for that hat! I want to wear it!
Tried to play one game of deathmatch and the loading screen after the host started was up for so long I gave up.
I also spent a couple hours with Charlie in party play free roam. Now, hopefully this is not the "co-op" of the game. It's just running around with a bunch of free weapons and vehicles. While we enjoyed ourselves, it won't have lasting entertainment value.
I actually don't think the co-op is working right now. I couldn't find an option for it on my cell phone(how you access multiplayer). The manual says to consult Rockstar's website about multiplayer. And their website under co-op says "Coming Soon."