Each car—and there are dozens of cars—can be improved by winning races, many of which have car components (tires, gears, chassis, etc.) as rewards. You do need to grab these in order to have a car that is good enough to beat the 10 boss street racers in the city. And, during this whole endeavor (except some races), the police are out there, just waiting for you.
The game takes place in one city. It is large and varied, with many nooks and crannies, but it is still a single city. This city not only has various districts, but it also has speed cameras (one of the goals of the game is to clock a high speed at each and every camera) and police. You can nudge a police car without getting into too much trouble, but if you speed or run a light or opt to hit a cop in a head-on collision (come on, it's just too tempting not to do), the law is going to be on your tale and you're going to have to escape them. Take too long to escape them and they escalate their pursuit with better roadblocks, spike strips, and faster vehicles. Not to repeat myself, but it's awesome.
I certainly question the physics of it all—some of the flips your car does seem relatively impossible—but in an arcade racer I'm not entirely sure that matters. You're playing the game because of the speed and thrills it provides, and it provides a whole lot. And, going online and playing others is a nearly seamless experience which really does actually make things more fun (as opposed to so many other titles where it's just kind of there). I am still not sure I buy into the concept of competing long-term scorekeeping with friends about who has posted the fastest time, but if that's your bag it's in there too.
The biggest snag to the whole title is this notion of it being a self-guided journey. The game will tell you where races are and what you'll get for winning, but the exploration needed to find cars is not something we regularly see. I keep coming back to Assassin Creed II's feathers that were hiding out in the cities. It was easy to find a bunch of the feathers, but after a while, hunting down those last few could be really painful. I don't necessarily advocate going out on the internet to find out where the cars (or feathers) are hidden, but you can and I'm sure many folks will.