Awesome game, but you need to get to at least 15% until you really experience it... maybe even the endurance races @ 25%
The A-spec mode is mandatory, but the B-spec mode is really fun too (especially for long endurance races because you can speed it up to 3x... making 24 hour races last ~8 hours)
From minivans, to Formula1, to Le Mans Prototype, to Gran Touring, to Snow Racing, to Toyota Priuses, the only things that this game lacks are graphics (pretty damn good considering it's Play Station 2... need to use Ps3 hardware to get better graphics), damage to your car physically and mechanically (they included this in Gran Turismo 5 because the Play Station 3 was able to incorporate a more extensive physics engine), and it could use more F1 cars/races... it is mainly Le Mans Prototype/Gran Touring cars/races when you get into the higher classes.