I'd have to say GT5 is good game, but underwhelming and even a bit lackluster, in areas. I understand why it took so long to create, but in the end, it really doesn't do much for me, after playing the previous titles.
A few of my criticisms:
Some of the same tricks work against the AI in races, and they've even gotten more relaxed on the requirements to enter many of them. I'm on the fence about how this affects the games balance, but this area could've used improvement.
The B-spec mode has improved, but the AI still needs work. My driver almost refuses to pass on the inside, despite clear opportunities to do so, and readily yields position when told to be aggressive, despite being ahead of the car attempting to pass. I'm hoping this improves as he increases in level, but he's 9 at the moment, so not exactly green.
The UI could use cleaning up-- they made progress with the ability to select appropriate cars from the race menu, for example, but it still needs work. Since the first GT game, I can't understand why they don't allow at least the option for a checklist/shopping cart of items for purchase, followed by a simple "purchase and install all" button. With the amount of cars available in the game, tedious doesn't even begin to describe how long it takes to add, then confirm, then install, then confirm, standard upgrades to cars.
I had to laugh at the oil change which managed to free 65hp.
I'm certainly enjoying the game, and will continue to play and let it grow on me more, but I can't say I'd rush anyone to buy it. I see some are rating it at 8.5/10? I feel that's an appropriate score.