I'm really not that into more realistic racing games (the only racing games I tend to enjoy are far more accessible ones like OutRun/F-Zero/WaveRace/Mario Kart/etc), but I started playing Forza 2 several months ago with friends, and now have Forza 3. I don't enjoy either game enough to invest much time in single-player, but the gameplay is solid enough that I really enjoy playing multiplayer (though I tend to choose a car of a class above my friends in order to be able to compete).
I particularly enjoyed Cat & Mouse in Forza 2 (not a proper game mode, but easily done with friends), but right now, I'm LOVING Forza 3's tag mode (specifically, the mode where you want to become "IT", so everybody is always chasing after one person).
I love just how customisable this game is. You can play seriously if you want to, but if you have a laugh and play some tongue-in-cheek gameplay modes, you're free to do pretty much whatever you want. For example, we played a game of tag in which the aim was to be "IT" for as long as possible, and everybody had piss-poor grip. We played on a massive open circular arena, where you cut cut people off easily, and it was a riot.
Not only am I loving the sillier gameplay modes, but as I already said above, the gameplay is solid enough that even a non-racing fan like myself can enjoy playing it properly.
Considering it's a genre I normally couldn't care less for, I can't say enough good things about the Forza series. Although I expect this to change in the coming month, right now Forza 3 is my GoTY for multiplayer alone.