I play Spy, Medic, and Soldier. When I get my spy on, heavies die and sentries die more.
Medic is fun because on the servers I play on, there's usually only 1-2 medic players on at a time. Of course, as soon as that demo sees a medic, he wants to be a heavy, along with the 3 demos standing next to him. We shortly get a wall of russiamen corpses and lose shortly after.