Top-notch in graphics and physics. Unpolished in the non-racing gameplay (menu's buying cars, experience system).
The physics are amazing. Its fun to take the Enzo Ferrari out, trying to control its monstrous rear end as you accelerate out of a turn. Even the slow cars are fun, they truly teach you how to take fast driving lines. The cars respond to tuning well, and you can really make the car change it's driving characteristics.
The game shines if you try to get golds on every event. It is very challenging to do so, especially the license tests and the "Special Events". It's just you, the cars, the track, experience points and money. The license tests and the "special" events are worth solely worth the money. I've played online a few times, it was good but not great. There's no matchmaking, which sucks, but it was pleasurable none the less.
If you are deciding between GT5 or Forza I recommend both, and just trade in the one you don't like. Gran turismo 5 is truly amazing.
Yes it isn't perfect....I would like to see some more balance in the career mode... I'm at level 32 and level 40 just seems rediculously far away. Maybe another 100 hours of gameplay away....