Well, measured by how much fun I have and will have with this game, I must give it a 10/10.
However, it is so heavily flawed in some regards that I can't actually believe it. It seems to me that PD focused on some points (car models, track design, driving model, feedback) and got that really right while they completely ignored others, namely those which were criticised in earlier GT games already.
My greatest complaints so far:
1. GT Life events. In GT4, people criticised that restrictions are too loose. You could just tune up your car and annihilate everyone with sheer power. What did they do about it? They actually loosened the restrictions even more. Most A-Spec events are completely unrestricted. While GT4 used to have at least tyre restrictions, in GT5 I can enter almost any event with a race-spec slick-tyre monster that once was a humble 458. The challenge then is not so much winning as it is to lap the opponents. It would have been so easy to make some restrictions, why on earth didn't they do it?
This is a drawback of earlier games made even worse.
2. The AI. Just as ever, most of them are just moving obstacles slowing down to walking pace in corners. It's telling that many challenges are about getting from last to first place in one or two laps. Also, they still ram you like you're not there at all. This is particularly bad at NASCAR events. One half of oppenents is pushing each other in a line, while the other half relentlessly rams you, making it more Destruction Derby than NASCAR. The real challenge is to be lucky enough not to spin out because some redneck asshole maliciously pushes the back of your car sideways.
And for some completely incomprehensible reason, you're given a disadvantage by making ASM mandatory, meaning that your car is constantly slowing down when not driving in a straight line. Why??
NASCAR implementation is actually so poorly made that it would've been better if they didn't include it at all.
3. B-Spec Mode. Honestly, who wants that?
4. Many of the standard car models, of course. Some look like they've been copied from PS1 games. Enough has been said about this already.
5. Poor menu design again. Why is there no "change car" option at race menus? Why do you have to exit and go back to the main menu if you want to only use another car?
6. Poor menu design^2. What was wrong with the arcade mode car selection in GT4? You had a country/make chart where you could quickly access any car that was unlocked so far. In GT5, you have a sideways list (which is a pain to scroll through) of some cars, and if you want others, you have to buy them and add them to your faves, only to make another impractical sideways list. Even worse, they have the tuning you did to them in GT Life. When playing with a friend split-screen, I just want to select some street-spec road car, and my friend selects a similar one, but not the same one. This has been made a lot more difficult without reason. Another huge step backwards.
7. Time trial challenge. There is no time trial option with online leaderboards and fixed restrictions like in GT5P. Why? Another step backwards, although one I hope they'll fix in a near-future update.
This all, and the fact that they released a boxed version without any online functionality in the first place, makes me think this is rather a beta version than a fully developed game. No other big title dares to be so flawed. Sometimes I wonder if Keiji Inafune was right by saying that Japanese developers are pretty hopeless in the present.
I really hope they get it right soon. If they continue to show no interest in improving the drawbacks, I might get driven off. I'm starting to get interested in what Forza 4 will be like.