I had gotten my copy around Friday night and had been playing for many hours, just messing around the city and doing some story (about 25% in with the story). These are some of the questions I have seen on the forums that I will try to answer, if you have any please ask.
The map is huge and amazingly detailed. It is not flat at all. It reminds me of when I actually went to Los Angeles. There is only Los Santos, no version of San Diego exists as previously rumored, also no San Fierro or Las Venturas. There are towns around the desert and mountains.
My only complain is the lack of off-mission interior locations. I think GTA IV had more interiors. I have only seen one enterable 24-7 store, no Burger Shots or Cluckin Bell like in GTA IV.
Handling: The driving is no longer like that in GTA IV. It is a bit more arcadey. When you hold down the accelerate button, cars burn out and accelerate like in Driver San Francisco (Although idk if that is the best way to put it, I haven't played many racing games.
Damage: Cars don't mangle quite as easy as they did in GTA IV, takes a bit more work, but in my opinion its pretty great. You hit another car really hard and that car's wheel might fall off or you might break its axle and the car gets crippled. You can fly out the windshield and at times it can be instant death. When you smash windows, it looks a lot more realistic and detailed now.
Flying: The flying can be a bit difficult, especially if your character doesn't have good flying stats. The most annoying part in my opinion is the camera when flying, it sticks behind you pretty much. When you do barrel rolls, the camera is focused on the plane and it can be a bit difficult to navigate (others might like this).
The shooting is like GTA IV with a bit of Max Payne 3. The cover system has vastly improved and to shoot you have to aim using the right trigger and shoot with the left. Not as easy as in Max Payne 3's run and gun but I'm not complaining. During combat you can press both right stick and left stick together to enable the special ability. Michael has bullet time, Trevor takes half damage and dishes out double, Franklin slows down time while driving and it really makes taking corners at maximum speed a breeze.
Definitely an improvement over GTA IV. Dodge, punch, kick, the animation is a lot better than GTA IV.
You hold down the left trigger and go into your fighting stance, people either curse at you, get scared and run off, or fight back.
It is harder to get away from the cops now. They are a lot smarter. Their shooting is more accurate. The key is to hide. You escape their line of site and you see some cops with a line of site on the mini-map. Stay out of it, avoid them in alleys, offroad, etc. You become wanted if you crash your car in front of a cop.
Characters and Story
I will be very vague here as possible, don't want to spoil anything. So far I have seen a few GTA IV characters return and some getting mentioned.
The character switching isn't always available. Sometimes if you accidentally drop into a character, you are stuck with a mission and can't switch back until you are done. Sometimes the characters are just unavailable.
Well you already know characters can be customized. There aren't many options for haircuts or facial hair for Michael or Trevor. Franklin has the most hair styles available and 2-3 beard styles. You can get tattoos. When it comes to clothes, for some reason Franklin and Trevor have the most customizable options. You can't even change the shoes on Michael, his clothes come mostly in a set (ie if you buy pants they will come paired with shoes). This is of course not accounting for DLC.
Paint jobs, wheels, body kits, suspension, engine, armor, window tints and more.
Different cars have different options.
I have not seen hydraulics yet.
LCS customs emails yous when they have new customization available.
You fly a plane and you kill birds if you hit them.
Pressing right on the D-Pad while near a pedestrian triggers some dialogue.
This is all I can think of for now. If you guys have any questions I will answer them.