This game lives up to expectations, so long as you're realistic.
I was fortunate enough to get my copy a bit early and thought I'd share my initial impressions.
Graphics: The graphics are stunning. I was expecting the quality to equal Gran Turismo 5 Prologue, but they are improved somewhat, particularly with regard to anti aliasing. Tracks look lovely, and even the notorious 800 'non-premium' cars look excellent. If these were the best the game had to offer, GT5 would be the prettiest racer. And yet the premium cars take it to the next level. Tracks are astonishing. Nurburgring is stunning, as the shadows from trees reflect off the suede interior of my Aston Martin DBS, and the sun sets, the line from 'uncanny valley' to simply 'you're really there' is crossed. I don't see how graphics can improve from this point and my eyes be able to detect it.
Sound: I have always complained that Gran Turismo's sounds are too synthetic and unrealistic. GT5 has made a huge leap forward. Change your exhaust system and you actually can hear it. I can tell if a car is a V8 or a V6. That magestic sound of a Ferrari's 12 cylinders is actually represented. I still hope the game moves forward. You don't get the throaty idle of an old pony car... but then you shouldn't be standing still in the first place. It sounds amazing on my 5.1 system. Lots of car sims support 5.1, but the doppler effect and different engine noises show incredible realism. I really like the BGM and menu music, but the game supports your own mp3s, which is wonderful because I will be playing this game long enough to want new tunes. The audio improvements also make the replays much more exciting. You will know what I mean when you watch some, though it's hard to really describe. You feel much more like you're at a race.
AI: It's great. I could swear that a car retailiated against me. GT has tended to have weak AI that drives in a row, and that is thankfully not the problem.
Physics: Some will not enjoy racing this series on its full realism settings. The game makes some effort to accomodate you, but actually being competitive with this game takes work. I think the physics scratch that realism itch many PC gamers have been demanding for years. I have not yet obtained the level necessary to see all the damage. That's one of the rubs of GT... you have to really work before the game opens up. There doesn't seem to be mechanical breakdowns, something I've always wanted to see in this series, but otherwise the realism is excellent.
Content: I wish there were a few additional car models. I think the problem is that GT5 has so many cars included that people get the illusion they should have every single car they can think of. It's so comprehensive that you actually are surprised when something isn't there. In other words, it's easy to be unrealistic and hope for everything... that's how much content you have here. There are so many historic and amazing racing cars in this game that it seems unfair to feel this way. There are many tracks in this game, though my favorite test track from GT4 is missing. Some of the weather and nighttime settings only work for some tracks, but when they do they are absolutely awesome. There are lots of things like photo album and setting up your personal area that just don't appeal to me personally. Online features are fully fleshed out, though, with very large fields of racers, chat, etc.
Fun Factor: I think a lot of people are going to complain about this. As gaming has become more and more popular as a form of entertainment, games have accomodated more players by being easy. I am not saying this is right or wrong: but understand this product is not an arcade game. You will have to have patience and practice to fully enjoy a more subtle experience. The fun comes from mastery. I think that's a lot of fun, but some people do not. The Kart racing, Drift, and to be honest, Nascar, present some cheaper thrills. You can also dial up the driving aids and have a very easy experience, but if you're looking for Blur or Need for Speed, this game just doesn't offer that experience. I do have one complaint: the game requires a substantial loading time, and the loading of new tracks took longer than expected the first time. One it's loaded once, it seems to save some of the content on the HDD, and it's much faster in the future. I have a huge HDD, but people who have smaller ones should be aware this game will need several GBs.
Value: I am stunned at just how much game is here. It is ridiculous. They could cut the content, tracks, and types of racing by half and you would still have two games worth much more than $60. I've only really scratched the surface, but I already can see this is going to be my favorite PS3 game.
My final thought on this: it's the comprehensive game for people who love cars. Old cars, new cars, fast cars, average cars. It's almost like am interactive museum experience for cars. It's also a comprehensive way to race with depth. Setup, driving styles, etc. It's like a racing school in your living room. The quality has exceeded my expectations. This game will impress you.