I bought GT5: Prologue last week, what a disappointment. There has been a lot of cries on the interwebs that it was just a demo an not worth purchasing, usually citing the limited number of tracks (6)and cars (60+). Which are in fact smaller numbers of cars and tracks from other GT games, but that didn't bother me to much, the demo from last year had a couple cars and one track. What I never noticed in any of the reviews or name calling was that GT5: Prologue is missing the one thing that makes a GT game a GT game, there is no tuning.
In a normal GT game the first thing you do after buying your first car is to go a put a chip in it or change the exhaust, usually the two cheapest things you can do to add more horse power to your car. In GT5:Prologue there is no purchasing of parts, it is just an arcade racer.
Once you get through the main game, levels C, B, and A the end credits roll and the S class opens up. On the S class races there is a tuning menu that lets you change the HP and weight of your car from a single screen. But this is just so you can try and get your car to match the other cars in the race, rather then build some obscene monstrosity with 1000HP and no brakes, which is where all the fun is.
One review I read said that the average winnings per race was only 30k, which means it takes a lot of time to get the money required for all the specific cars you need. This isn't actually correct, the average per race is closer to 10k, only the S class races give you 30K, which you can only get to once you are finished. Probably the best race is one with a 7.5k pot since you can win it in under a 1:10. But it is pretty hard to win so normally it is 5k a run. Rerunning races is very quick so there is minimal downtime to do it again. There might be a couple of races that make more money if you are a better driver then I am, but when you are grinding cash easy races are the best.
What is there is entertaining if you like driving games. The online races seem to work ok, there is a nifty feature where if you are out of control your car phases out as to not mess up other drivers which fixes most of the griefing that can happen in a racing game. But keep in mind it is more of an arcade racer then the prior GT games. Which makes the inclusion of an arcade mode even strange since they are the exact same races as the normal races you just don't get any money for winning.