IGN put it best: 10/10 simulation in a 5/10 game.
:: Sound -
-Music: 3/5
So the in-game music isn't awful, there's a reasonable choice which you can customise (I opted for classical music while driving which felt quite classy), but the music in some bits of the menus is atrocious (the customisation menu music springs to mind).
-Sound effects: 2/5
Bad. I guess the engine tones are fairly realistic but they're flat,unexciting and boring. Take the Gallardo, which has a ferocious V10, in-game it just doesn't have that exciting roar that the real engine has.
:: Gameplay -
-Graphics: 3/5
You may be thinking "Whaa? Surely GT5 has awesome graphics?". When you've got a premium car on a premium track, sure. But thats only 1/5 of the cars and about 1/4 of the tracks. Otherwise it looks damn awful, obviously ported straight from earlier GT games.
-Car selection: 3/5
OK, 1000 is a very impressive number. But so many of them are either really old models or are slight variations of other cars. The game features 50 nearly identical Skylines and maybe 20 Mazda MX-5/Eunos Roadsters. The roster of newer cars within the premium models is good, but because all the standard cars are from older GT games, they're all the old versions of different cars.
-Physics: 5/5
Outstanding, truly brilliant and uncompromisingly realistic. A real highlight of the game.
-AI: 1/5
Pretty much non-existent, the cars basically drive on rails and are pretty characterless and boring to race against.
-Difficulty: 2/5
Completely messed up difficulty. Although you can adjust the assist settings, there's no difficulty for the AI (testament to how poor it is). The game also has a terrible habit of starting you in last place in every race, meaning the only way to win is either to have absolutely beastin' skillz or to start with a better car, which just feels like cheating.
-Career: 3/5
A big check list of races to slowly work your way through. Pretty par for the course for this sort of game, granted, but the aforementioned AI and difficulty problems drain a lot of the fun out of the races.
-Free play: 3/5
It can be great fun to race in GT500 race cars around the Nurburgring or Nascars around Daytona, but of course it is hampered by the bad AI, difficulty and also the fact that you can't pick any car from the list of 1000 to take to the track. You can only pick a car you own in career mode or from a pre-set list of about 40 premium cars.
-Damage: 2/5
When damage is at maximum it looks good on some premium cars, fantastic on the rally cars, but is almost non-existent on standard cars. But that isn't the problem, the problem is that Kazunori Yamauchi made the fantastically bad decision to make damage locked until you're most of the way through the game. Not only that, but the game never tells you this (that's why there're loads of people running around on the internet saying GT5 doesn't have any damage at all).
-Upgrades: 4/5
Pretty good, you can upgrade almost everything (except brakes, weirdly), but it has the same issue as all other sim racers of this kind: upgrades lack any kind of side effects, other than increasing power or grip. For example, when I fit a huge turbocharger, I want it to adversely affect engine cooling, such that the engine will kill itself unless I fit a better cooling system. Many real upgrades do have real negative side effects like that.
-Controls: 4.5/5
I used a Logitech G25 with this game and it was superb, the best force feedback in a racing game I have ever felt by far, I mean you can actually feel the road surface through the wheel. I only knocked half a point off because the clutch isn't properly supported, and acts pretty much as an on/off switch, but it isn't a huge deal.
-Special race mode: 4/5
Some great races to be had in this part of the singleplayer, Nascar and Karting were great fun but a particular highlight was a series of races starting in Austria and going down through Italy through Tuscany and Rome. The races are, however, hampered by AI and difficulty issues.
-Photo mode: 5/5
A really great mode actually, you can play with the camera settings to get some lovely looking shots. I can't help feeling though that the effort put into this mode helped to drain resources from the main game.
:: Interface -
-Load times: 0/5
Horrendous, even with the install they are intolerably long.
-Menus: 1/5
Terrible menu design. Laberinthine, poorly thought out and confusing to find something you're looking for. Full of bad design choices.
-Options: 2/5
It feels you have a general lack of control over things. For example you can't change the AI difficulty settings, and why is it that when doing a freeplay race you can't change things like whether it's a rolling, standing or staggered start?
-Choosing cars: 1/5
This is probably the most annoying thing in the game. I've awarded a point because it's easy to sort through the cars in your garage. However, the way I see it, games like GT and Forza are like car encylopaedias. Most of the fun is in buying and driving all different manners of cars. But in GT5 you can only drive a car (either in career or freeplay) if you own it. Premium cars can all be bought as expected, but the remaining 800 cars can only be bought in the second-hand car dealer. This is one of the worst design choices I have ever seen in a game. So a random selection of about 30 cars is chosen for the dealership. After every race you do, the oldest 4 are removed and 4 random new ones are put in. Since so many of the standard cars are all just cheap Japanese sports cars, these seem like the only cars that ever come up. So if you want to drive a specific car, say the Bugatti Veyron, which is I think a very understandable desire, even just for one quick lap, the only way of doing this is to wait until it just happens to come up in the used car dealership. And even then you'll need enough credits to buy the thing.
:: Summary -
But you know, you can pull a game apart and analyse every little bit but in the end it's the overall feel of the game that leaves the lasting impression. But Gran Turismo 5 has the feeling like it just doesn't come together as a cohesive package. It's like a huge unorganised splat of content that cannot support its own weight. It seems through development Kazanori Yamauchi was so keen to add on more and more extraneous content (I remember him saying he wanted to add a feature where you could record a real lap in real life with a satnav and put it into the game as a ghost lap) instead of focusing on fixing and polishing the core game that he already had. There are a few sparks of absolute brilliance within this game, some truly lovely moments. But it's all buried deep beneath poor design choices, and as soon as you feel like you're getting into it an AI driver slams into because he is totally unaware of you or you are forced to wait 3 minutes just for a track to load.
Final point: GT5 didn't just have incredibly high expectations to live up to, there's the elephant in the room: Forza. Since playing and selling GT5 I have bought an Xbox 360 and bought Forza 3, and god it blitzes GT5 in every way. It may not have the physics or the steering wheel support, but every other respect of the game is so much better thought out and implemented. Everyone thought the 'king of racers' would make a triumphant return and see off the dastardly Forza 3 who had stolen his throne, but the old king is going grey, fat and is starting to smell weird and we've come to realise that life with the other guy was a hell of a lot better.