Great so far. I've put about 8 hours into it since Tuesday, but am predicting a HUGE gaming weekend. Should be up around the 30 hour mark by Monday night.
Its "always online" so you always can jump in and out of co-op multiplayer in any mission you are doing. I found it very challenging when trying to do story missions alone, but still doable with a few attempts.
I've done two or three story missions in a "squad" and it is crazy. The fire fights are huge, and people actually work together in this game. I don't talk to strangers online, but it seems to work without any real communication anyway.
You can tell when someone in your squad is down, and help them when they're in need. The good thing is people actually do try to help you in this game. A lot of other games, you're together, but everyones playing for themselves.
People also seem to know some good combat strategies, like flanking, and drawing fire one way so someone else can hit them from behind (
![Wink ;)](./images/smilies/icon_wink.gif)
![Wink ;)](./images/smilies/icon_wink.gif)
I'll chuck some screenshots and videos up later tonight.