Project Gotham Racing 4 is an absolute masterpiece despite some unfortunate issues.
The most striking things about PGR4 initially are both the customizability of your driver, and the quality career mode. In the game, you pick both the gender and nationality of your driver, as well as their outfit colors. It's nice to see a racing game that adds this element - the ability to have some part in the creation of the character you'll be driving around as. It's not detailed, and doesn't really waste your time for any more than a few seconds, but it is nice that it's there.
The Career mode of PGR4 is quite nice. As you race in your career you will collect "points" and Kudos. Points accumulate and are used to advance in rank in the world against AI drivers. You gain points based on your standing in a championship - the better you do, the better your points, and the faster you will advance. You keep doing this, advancing in rank and unlocking harder championships, until you reach the top. As well, the game doesn't actually end there, so if you like the feeling of making a character and getting them to Rank 1, you can continue to accumulate points forever. The career technically does not end.
Kudos is one of the main focuses of PGR. Kudos is a reward system you earn by doing manuvers. These manuvers include drifting, powersliding, high speed, skilled "proper" turning, passing other cars, etc. For bikes you can earn it for many of the same things, as well as for things like wheelies. You use Kudos to unlock various things, such as tracks and vehicles. This is used instead of a traditional "buying". You purchase packs including multiple tracks, vehicles, etc, with Kudos instead of buying them singly with cash.
The driving engine in PGR4 is a lot of fun. While not a realistic driving model, it does have a bit of a feeling of a very forgiving but solid feeling model that blends arcade racing with simulation racing quite well. On top of that, instead of merely doing racing derivatives (timed laps, eliminators, street racing) there are also some unique races that use both kudos or precise driving skills very well. Superstar is a good example, as it is a game that uses the Kudos system to it's fullest.
The game does have some bad points of course. The game seems a bit too hard, especially since it tends to clump factors that would make the game difficult together (e.g. Nuremburg, the hardest track in the game, in the snow, the hardest weather. Or doing races in the rain at night.)
In addition, the Kudos system seems a little unfair, as you can often lose Kudos for crashes that aren't the fault of a manuver you were attempting. It seems to take a long times to count Kudos (sometimes 10 or more seconds after the manuver is done). There are other issues, both those are the most noticible.
Overall though, the bikes, the weather, the tracks, the incredible visuals, and the career mode all come together to make this game awesome even with it's flaws.