Minor spoilers ahead!
I just finished the game. It's pretty good. Haven't done many side activities yet, mostly just story missions. Before you make your final decision, I suggest looking up the consequences for each, because there definitely is a right and two wrong ways to finish the game. I picked one of the wrong outcomes, but luckily, I can still reload from before the final mission.
The story constantly switches between serious and ridiculous. It can't decide what it wants to be. Basically, everything you do is an epic fail and you always find yourself in situations where you are surrounded by entire armies of thugs/military/police/mercenaries and would be dead, if it weren't for your guys being superhumans. It's not a big deal and it has always been like this in GTA, but for me, it's a bit of an immersion killer, if you see your characters acting like mortal and sensible people all the time and then survive one impossible suicide mission after another. But hey, the game is trying to be serious while letting you slaughter thousands of professional gunmen at the same time, so this kind of inconsistency is a given.
The characters are mostly annoying stereotypes and everyone is a hypocrite, especially Michael's intolerably self-righteous poser of a son. Trevor can also be annoyingly self-righteous. For being a violent, murdering psychopath, who's only way of communicating is intimidation, he is extremely preachy, especially towards Michael. But since Trevor is not pretending (or even trying) to be a good guy, I guess that somehow makes it ok... The other two main characters are pretty much your average GTA mass-murderers, who do what they have to, even though they don't agree with most of it.
In my opinion, being the crooked cops' bitch again (like in every GTA), played too big a role in the story and tbh, I found that aspect a bit hard to follow.
The heists, although being well-designed missions, are a bit disappointing. What makes them stand out is that you can choose from two different approaches and have to do different preparations for each. The crew feature is not even worth mentioning and just forgettable.
The driving didn't bother me as much as I thought. Sure, the handling is too easy most of the time. That also means, that police cars are often hard to loose. The feel for each vehicle is pretty different, however.
Sadly, Euphoria physics are used less than in GTA4. They are still there, but everything feels (and probably is) more animated than before. You won't see people loose their balance after a punch and drunk-mode just gives you a different walking animation.
The stock market is not that good either. The worst thing is that the individual stock charts are all fake. I tried making a profit several times by buying stocks with much fluctuation at their lowest and then wait for a better price. Only the fluctuations suggested by the charts aren't real. The charts are just there to confuse you, apparently. Stock values just stagnate most of the time. There's little change. The only major changes derive from the story. Sometimes you get a hint where to invest. Using those is the only way to make a decent profit on the stock market, as far as I know.
The world is huge and varied. There are many random events and side activities. Mission checkpoints keep the frustration level low compared to GTA4. You can buy and upgrade tons of stuff (although you don't really need to).
I was hoping there would be some San Andreas-like "cheats" - not for armor/weapons/vehicles, but for fun stuff like arming civilans with assault rifles or disabling gravity. Like in GTA4, there isn't even a cheat for disabling wanted levels. So whatever mayhem you want to cause (before likely reloading the game), everything turns into a cop chase pretty much immediately and you can either run or fight to the death.
