For what it's worth, more impressions after 23 hours of playtime in Online.

I have not come across any situations where players ruin it for others. There was one occasion when someone appeared to be sniping me while I was hunting (he missed but I heard the bullets whizzing by me) and also a hilarious moment when someone mugged an AI character and stowed him on my horse while I was in a shop. Also in the story missions, most people I have come across seem to be making an effort to team play, or at least not interfere.
Unfortunately many missions are on a timer, which practically forces players to rush in headlong when a more tactical approach would be sensible. I realize this is not ARMA, but I would like to see teamwork rewarded. Missions you play on your own (deliveries, bounty hunts...) are also on a timer, which in my opinion is a lazy way to set a challenge. I am doing the Bounty Hunter and Trader roles. Trader seems more interesting as it is linked with the Moonshining business but I am not sure if this will keep me interested in the long run. I can do all the hunting and fishing I want in Single Player without having to worry about server disconnects (I had a few of those) and, for me at least, multiplayer does not add much to that experience.

The few Free Roam Events I took part in were very much as I expected: every man for himself and prepare to die within 30 seconds.