Well, as you talk about popularity this awesome game has his followers and it has its cool deals.
You have the initial part, where you race through all the stages having the chance to enjoy smooth riding and at the end, they give you a beauty of car, the McLaren F1. This you can't buy as the other ones, this one you earn it just by finishing the game. Some hate it, some love it!
Then you have UTT (time trials) where you play to beat yours and other people's lap times which is very cool. Here you play in a track of your choice all empty and for youself. No distractions, since there's no racing here and it's real nice as a tool for improving your performance. It's kinda best lap trials at Formula One, except that it won't guarantee a first pole position in a real race at OMP (Online Multiplayer), but it can give you the chance for a good race and winning because of the training.
It also let you play OMP racing (online multiplayer) at different levels and according to your mood or how you're feeling at the moment. You can race any car you want:
A low end engine will give you the chance to practice your just acquired racing skills, like at 94p Tier by example. Calmly you can find your breaking spots on the tracks, adjust your everchanging lines perception, practice passing other cars in a real race. Also, you got the time to look around and check out the beautiful scenery besides the chicks in the galeries, your friends and such. You won't have to worry about crashing other cars unless you're blind of course, but be careful, sometimes you can fall asleep at the wheel because a race take forever to finish. Other than that it's very cool, since it's an extended and transitional training level to prepare you for 125p or a place to come back and relax after the battles in the wars of a higher Tier.
A high end engine will give you the chance to get down and dirty in 125p Tier, where you can prove yourself and your training under the best scenario. You have there a stormy day every time you race. That's where you feel the heat, the passion. You'll encounter lots of testosterone, guts and glory, the tough, the speedy, the reckless, smoking rubber, screechess and crashes. You'll find there the fast and the furious and also dumb and dumber! Trying passing somebody at that speed and be my guest, wish you luck with the rest in front! But if you can find peace inside the storm, you got it. Nothing like the calmed rush, priceless! That's what the best racers there have found. They are the cool brains, the ones that know how to control themselves to find a win in the middle of all that chaos. The cold minded. You can also choose your engine size as they come in all colors and flavors. I recommend using lower performance cars regularly for purposes of cooling off and regroup yourself after a crazy sesion, nothing like that to calm down and re-practice your performance and they're still good for the first five spots in a race if you're in the mood.
So, there you have it, try all phases of the game, choose what you prefer and have fun!