First things first...if you're looking for an easy racing game, then don't bother buying it - GT mode is a simulator, NOT a "pick up and go" arcade game (although there is an arcade mode).
The handling of the cars is extremely realistic, especially the engine sounds - polyphony have made the effort to make all the cars sound as true to life as possible.
GT mode, although daunting at first given it's magnitude, becomes a way of life. It's fantastic. The array of cars is superb, tuning, used cars, with races starting at beginner mode right up to extreme mode.
I have to reiterate that this game is designed to be challenging, the licences are difficult as expected - there are 6 in total, amounting to 60 challenges/tests. It encourages you to re-take tests to shave those times to earn gold. This is a simulator - you wouldn't be able to jump in these cars in real life and get gold first time would you? You need to get used to the cars characteristics, and see what it can and can't do - every car is different. It requires you to tweak settings to get the optimum balance.
This game as a hole is light years ahead of the competition, and is the closest to reality simulator out there.
3D mode is also good, the depth and perception it gives you is fantastic - a nice little addition to the game.
Online, although buggy as first, now appears to be almost perfect - I have had no issues playing with friends. If they are playing online, just click on them within the GT community and enter their lounge - it's that simple.
Also the damage is very very realistic, but you have to earn it - the more you rank up, the worse the damage - a good way of doing it, as you progress your skills. Level 40 gives you full damage, which leads to bumpers, bonnets etc flying off - the works!
This is a huge/long game, which will take hours of your time to complete, but is also very rewarding and in time becomes a religion!!