The DLC is great, is it worth $20.... no. But if it does go in sale it is well worth it. the game provides a few interesting new features, such as...
1) Ice- (makes you go "i got thi... OH SHIT")
2) Water slides- do LITERATELY nothing
3) Magnetic track- allows you to go upside down.... woooooooww sooo coool (sarcasm if ya couldn't tell)
4) "Anti-Drag" fans- they're just normal boosters
5) Boosters- there are like the normal booster that I just mentioned, if those boosters where on steroids and did crack
6) New Cars- the cars that came with the DLC look GREAT, they bring a fictional feel to the game. and in a way that doesn't overpowers it (my FAV is this one car twin v8 supercharged engine)
CONS:
1) AI- the AI is very inconsistent. you'll be racing normally, or you'll end up with the SAME EXACT car next to you, with the same rating, yet somehow they pull a few hundred HP out of their ass and beat you by 7s. like where the hell do you shave off 7s? but don't worry sometimes the AI goes "handy cap" mode and run into a wall. I've only seen this happen a few time so don't bet on it.
2) Progression- I think the progression system is cool, and makes sense. but one thing I hate is some of the "missions" forces you to get to point A to point B. That part doesn't sound that bad, but it's a little hard if you don't know where point A or B is. For example, in the game have drive from the "Hot Wheels Outpost" (the middle of that map) to "Hammer Mountain" in X amount of time. keep in mind I don't know where the hell Hammer MT is. So I'm driving around for like 2 hrs, driving at 60mph, cuz all the fast cars I have are locked because the DLC forces you progress by doing these DUMB FU..... sorry, lost my temper.... these missions.
But other than that, the game is great and I recommend to all the pre-existing Forza players, and all other racing fanatics