I love this thread, there are some great ideas and discussions.
After finally getting around to buying Toca 3 for the PS2 - the XBox version sucked so bad I barely touched it, I'm finding my experience pretty similar to JohnBM01's. It's great, but aggravating as hell. For two reasons. One, the bots do get more agressive as you progress, stupidly so, almost as bad as in Forza. Two... well, this requires some griping.
Some things are scripted in the game, both in the physics engine and races themselves. Some races, you utterly blow the bots away with a 20 second lead. The next race, unless the game has a brain cramp, you just cannot win. The bot cars also don't obey the same physical laws your car does, so they can bash you off the track or spin you out at will. Do over, do over, do over do over do over... GAH!
However the thing does have its good side. It's very deep, with almost TOO much to do. Heck, there's even lawnmower races! And the wealth of race cars and tracks is heavenly, overshadowing the hellish grind of racing. And the Professional Mode with its many careers to choose from is incredible. Having all those cars in race, and the vast number of race courses from around the world gives it the awesome feeling of being involved in a real race event.
This is exactly what I've been calling for with Gran Turismo 5. Give Career/Professional Mode its own world, while still giving the pick up n play gamer GT Mode and Arcade. That way, those of us who want to go to the next level with Gran Turismo can have a career experience, and when we want to just go to a track and race for a prize car, that's there for us as well. Having a Career Mode won't affect the flavor of Gran Turismo at all, as it would still be based around the old school GT we all know and love, allowing us to be would-be billionaire car collectors. But when we wanted to enter a career world of our choosing, it would be there for us.