First impressions from what I've played:
Character creation was interesting, creating characters based on heritage is a real interesting concept.
The game is seemingly seemless, in the way of who you can interact with and what you can do. Its so similar to the Story.
When I was looking for players to race with, it took some time, but eventually I got a person and we race. How you create your characters lifestyle which infleunces stats really show. The guy I was racing with smoked me, but when we did a drug running mission, I was the better shooter. Cutscenes look great as well. Went to a cloths store and there was alot of variety of cloths, even more so then the singleplayer, though some cloths are locked until you get to a certain reputation. Car customization use the same principle, with items locked until your RP is high enough.
While I would still be playing, there are expected hiccups (alot more then I would expect) so connecting to Free Roam is a pain. When I would connect, a couple minutes later I would disconnet or basically have my screen freeze. This could cause alot of issues with a players character profile. Like I customized a car, exited out the shop then it froze. Tried to connect after multiple attempts, and I do think the game saved in time after I customized my car, though I still have to find it.
Overall, GTA Online is great, the progression and structure and things you do in this game is conceptually great. Once the server issues are rectified or whatever's fixed that makes the experience better, GTA Online is going to be amazing. If you thought Free Roam in GTA IV and Red Dead was fun, you'll love GTA Online.