So far I've played a bit of it and I'm impressed. Will play more tonight. I particulary enjoy the race track in Switzerland, but that should come as no surprise.
Also throwing cars you've driven in real life into corners you've taken in real life (comparable corners at comparable speeds) is always a blast. And throwing cars into corners is not easy in F4.
In fact I like the heavy, yet very controllable handling of the cars. Like, well, a car. The graphics are beautiful, albeit a tad bit too shiny but then again I never cared about graphics in a racing game (especially since my all time favorite racing game is Need for Speed III from 1998).
I say get Forza 4 if you have the chance, and get the COllectors Edition - the Artbook is really nice. And its incredibly hard to distinguish between ingame screenshots and real photographs.
Also, Top Gear. Nuff said.