A few first impressions, and also my viewpoint on the series as a non-fan.
The GTA games are of a type that generally appeals to me in concept as far as gameplay goes, but I've always found something of a turn-off when it came to characters and controls. The 3D ones so far I felt were particularly clunky, and I could never get past that. As for the characters, well, this may sound a bit wet but I do have a genuine problem playing criminally-inclined bad guys in games. I did the Light Side in KotOR, the Paragon path in Mass Effect. Unprovoked viciousness doesn't appeal to me, so a lot of the guilty pleasure in GTA is lost.
This one seems to have fixed a lot of the things I had problems with before. The setting appeals to me (it's essentially New York) and the player character is more of a guy in a tough situation trying to get by. He has a weariness about him, and he certainly doesn't seem to take much pleasure in the dirty business he has to get up to. Not so far, anyway. I'd love for Rockstar to have pulled off a game where the player is confronted harshly with the kind of actions they've traditionally pursued with glee. I have a feeling it won't go that far, but it does certainly appear to be more grounded in reality than previous efforts.
The city is magnificent, and frequently breathtaking in its vistas. The weather plays a large part in that. Golden sunsets are aplenty, and frosty mornings almost chill the screen. The density of detail is amazing. Look closely enough and you'll see plenty of repetition in environments and NPC behaviour, but you do have to be searching for it to notice.
As usual with these kind of 'sandbox' games, I'm more inclined to just exist in the world than get on with progressing the storyline. There's plenty of time-wasting to do, from hanging out with friends to going on dates (and the various activities you can choose to do there), to simply sitting in and soaking up the staggering array of parody TV shows and internet sites. It's very funny as well. Sometimes completely crass and sometimes very subtle and clever. Just aimlessly driving around to listen to the radio is a pleasure that can soak up entire sessions.
Early days in and Nico is just a small fry, with limited access to the city. Considering how impressed I've been with the area I've been given run of so far, the prospect of what opens up later on is very tantalising indeed. It's not perfect by any means, and thematically it doesn't appeal nearly as strongly as other types of adventures, but I am having a lot of fun with it.