I find this game to be the best in the series, and I've played them all.
While not only racing you can also free roam. Not that the racing sucks though it is still a major part of the game and is required to advance.
bosses-There are 4 bosses. Two are of the same level one is incredibly easy and another is incredibly hard. Each time you beat one you take control of all their territory, but you have to wait for them to challenge you. This only happens when you have taken most of their territory. Also when you beat them you get the choice of 5 things 3 are a mystery the other two really don't do anything. The mysteries are pink slip(you take their car), get out of jail(I'll explain later) and a cash reward. There comes a time at the end of the game where you have to beat 3 of them again and this time its really hard.
beating a boss- to beat a boss they have to challenge you. the first part of the challenge is the city race(its just like any other city race in the game except its just you and the boss). the second part is the canyon. you can lose several ways as well as win several ways. you can fall to far behind, you can fall off the road. you can win by getting ahead of them for 10 seconds or pushing them off road. there is also a second part to the cnyon where you start out in the lead and the winning losing situation is reversed.
gang- you are the leader od a racing gang. you can have 3 members as well as 3 types. the first is the scout(finds shortcuts), blocker(screws up enemies), or the drafter( allows you to draft behind them)
races- there are several types of races. there are drifts, normal, challenges.
cops- cops really break down on you hard. the more crimes you comite the harder it is to evade. If you get caught they through you in jail where you can either use a get out of jail card or pay. they also use road spikes road blockes oil spilss and so on to catch you.
my opinion-I love it. if you like racing screw renting it. awsome game.