Just had my first, true, impromptu GTAO moment:
I had botched the dumpster Unique Jump at Los Santos Airport, for maybe the 5th time. As usual, the cargo helicopter was on the tarmac, so I bagged that. I wondered if I could get the chopper's cargo hook into another character's car, so I flew off to find out.
When I arrived near a group of players who were pursuing and killing each other, one of them was in the police chopper with machine guns. The cargo helicopter is unarmed, so I started flying and bobbing and weaving around buildings to not get shot out of the sky. I managed to put it down on a downtown street, get out, and take cover. The other player was still trying for me, but I managed to lock on, as I was on foot, and took him out. I grabbed his chopper and circled the block in it, then landed right near him. I thought he'd probably just kill me, but instead, he used his Salute action. I Saluted back.
We jumped in a nearby car, and sped to a Race initiation point. It put us smoothly into the same lobby, and we had a kick-ass race (Lost and Found, I think?), which I barely managed to pull off.
It was pretty transcendent for me. This is everything I'd hoped GTA IV would be, 5 years ago, and then some.