I got my copy of Forza Motorsport 3 last Tuesday, along with Tekken 6, and the former game has taken up most of my time. I will have a review up over the weekend. In the meantime, I should note a few things. 
 
Buy the game new so you can get the Benchmark high-speed ring. Whipping around that track in a Bugatti at 240 MPH in the cockpit view is the best experience ever in a racer. 
 
Also, all of the cars are unlocked for use in quick race and time trial modes, which I like a lot. It's great to sample the supercars this way. 
 
The cockpit view in some of the cars is claustrophobic.  
 
There's a few Days of Thunder replicars on the storefront. I need to save up, get the Chevy Stock Car and get the #46 City Chevrolet livery ASAP. Talladega Nights: The Ballad Of Ricky Bobby is also well-represented. (No Jean Girard or Cal Jr. cars yet, though...) 
 
Achievements are handed out like candy. In the two days I played the game, I got 20 achievements. Achievement whores will find a lot to acquire early on. Don't know about the late-game achievements, though. 
 
So far, so good. Well worth the $$$.