Easily the best racing game ever made, hands down. It's true that there is not a whole lot new over Forza 3, but given that the game was already pushing the max of what the 360 can handle graphics wise there really doesn't need to be. Everything is just a little bit more refined and has a few better options.
The amount of cars (and the detail on each one) is staggering. If you drive any kind of sporty car, there is a good chance it's in here. Even the exhaust sounds dead on for the different models. So if you ever wanted to see how fast your car would go around the Nurburgring (or better yet, the Top Gear Test Track), this is a cheap way to do it.
The addition of Top Gear is what really improves the game over the previous. The opening movie narrated by Clarkson sets the perfect tone for this game. Having the background for your cars be the top gear studio is awesome. Clarkson also narrates a few of the cars in the Autovista showroom too, giving full on Top Gear style reviews. If you are a fan of Top Gear, these additions to the game are huge and feel just right.
I suggest using the Microsoft Wireless Wheel, it really adds a nice feel to the game. It is the first motion control I have ever used that actually improves the control you have over the vehicle. It makes things like counter steering feel natural and smooth (something very important if you are driving any of the muscle cars).
If you are a gearhead, this game is simply a no brainer. Even people with a casual like for racing games will find a lot to enjoy here. There is no question this game will reign as the supreme racing game for this console generation.