Age-old exaggeration. You come across them once in a while. In fact, its a better time than ever to get online because most of the people still playing are the bigger fans who enjoy some good racing.
Some people are unlucky and have their first couple experiences online ruined by some asswads and never go back, but the highs outweigh these lows. Especially if you use the user-created lobbies, its not hard to find a good room full of clean racers(or at least ones who attempt to be clean, accidents are inevitable) who also use their mics. And this is where Forza really shines as one of the best online experiences out there. Forza 2 was very big in creating a great community because it was ALL user-created lobbies and those were great times. Forza 3 made some huge improvements in other areas, but went strictly matchmaking for public multiplayer, which wasn't nearly as good. Forza 4 has the best of both worlds and also has the brilliant Rivals mode, which provides a ton of fun.
I really think people who focus on single-player in these games really miss out on the best aspects of them.