I played a little last night. Still seems buggy, but it's neat. I can see it being heaps more fun with friends than strangers. I'll have to get on with the gtag crew at some point.
At the start where i had to steal a car to get the tracker in it, i just got a shitty one so i could drive around and look for a better one. But as soon as i got that the streets were literally deserted. Not an NPC or parked car or anything in sight. I didn't go find another player to see if there were some around them. But i was driving around for ~5 minutes and there was exactly 0. I went into a clothes store and bought some shorts and came back out and it was back to normal again, so that was weird.
Then not long after I tried to go do a Last Man Standing deathmatch thing, and it started loading up fine, but just kept loading forever. I waited over 10 minutes but eventually had to completely close GTAV. It had loaded something because i could press up and down and it would click like i was scrolling through a list, but nothing came up and there was no way to exit that mission. :/
So I went back to Singple Player after that. There's still heaps i have to do in SP.