I'm hardly a sim racer. I enjoy driving games and use a mainstream wheel (Logitech G29), but I'm not all that interested in competitive racing, or overly onerous simulations.
The original Assetto Corsa was a pretty good experience. It did suffer from several inconsistencies, but overall, it was a top notch sim racer that was also relatively accessible. With Assetto Corsa Competizione, Kunos takes everything to the next level. This game was in early access for a fair while, and has become essentially a live service game. Initial reviews pointed out some glaring issues and omissions, so I waited till the game hit version 1.3. My review, thus, is based on my experiences with a recent version of the game, which is much improved since the original early access and even the official release.
It's a very different game from the original. ACC is laser focused on quality over quantity. As the official game for the Blancpain GT Series (now simply the GT World Challenge Europe) that's all you get - a few GT3 cars on a few tracks this series visits. That'll no doubt be disappointing for sim racers used to messing around in a whole variety of cars over several tracks from around the world. On the other hand, it gives the developers the chance to fine-tune the experience to perfection for the limited scenarios.
And that, ACC does remarkably well. Once again, I'm hardly a sim racer, but to me, this is the most realistic driving a car in a game has ever felt. The amount of detail and granularity is staggering. Each car feels different and distinct, with their own subtle characteristics and traits. I've never played a game which really showcases each car's personality quite like this. Of course, with all cars following the stringent GT3 regulations, the overall feel is similar, so you get to grips quickly.
Some sim driving games have the tendency to become "understeer simulators" and such - basically make driving harder than it's supposed to be, for the sake of challenge. Not so with ACC. GT3 cars have an incredible amount of grip, and as a result, the game is actually pretty easy to get to grips with. Of course, the real challenge with racing is to maximize the speed - to be right on the edge. ACC simulates this concept wonderfully. It's easy to get complacent with so much grip and downforce, but the moment you start pushing, it can get hairy very quickly.
The move to Unreal Engine 4 has meant far more detail on both the cars and tracks. The car models are some of the most detailed I've seen, especially the interiors. Outside of the cars, the environments are nothing exceptional, but does bring the series up to modern standards. One complaint is that it doesn't have a proper anti-aliasing solution. There's an experimental one, but it isn't on par with the Codemasters games' TAA or Forza's performance-optimized MSAA. As you would expect, the sound is phenomenal. With headphones and the right tuning, it almost feels like you're there! It's the little details that matter, and that's reflected in the sound as much as pretty much every aspect of the game.
So, the driving experience is quite possibly the best I've seen in a game before. Unfortunately, it just isn't a good single player experience. The Career mode is bizarrely poorly thought out. The sense of progression isn't rewarding, and the way it's segmented into sprint and endurance is baffling.
The endurance races are where ACC truly shines, with perfect tyre, fuel and other simulations really making it a long, strategic battle. The single player experience, once again, suffers due to a rather underwhelming AI. It's clear it wasn't a priority for them and it shows.
After finishing the career mode, I was left unsatisfied, so I did a full championship separately. This was much better, but ultimately, the developers simply didn't think of crafting an engaging single player experience.
I know - this is a sim racer - the focus should be on the multiplayer. That's fine, and the multiplayer indeed is very good. But I'm not a competitive gamer, I want the fine-tuned single player experience. Unfortunately, ACC just cannot deliver a compelling one just yet.
If you're a competitive sim racer, then you've already played ACC. If you're more of a regular gamer, should you play ACC? I'd say, yes - the game's core driving mechanics are so good it's a must play for anyone interested in driving games. Consider skipping the career mode, though, and set up your own custom championships instead. I do hope that in a future update the devs will release a better thought out career, but that's just wishful thinking at this point.