I can honestly say the only thing I noticed about this game is the sound. All of the imported sound tracks drop about 50% in volume (even when setting race volume to 120%), so I have to turn my TV up (not like its a big deal).
Other than that the cars sound just like they all did GT4.
And as far as sound goes, theres no way to manage or even check which audio tracks you've imported off of your PS3. I just went down the list and imported all of the ones I wanted to hear while driving. Now there's no way of actually seeing or even REMOVING the ones that I've imported already. Ugh. Thats my BIGGEST complaint about this game and its not exactly a big one.
I haven't got a try online yet, but I'm having too much fun trying to get my licenses.







