I dabble in almost all classes to a greater or lesser extent (except scout, where I just die!). And I'll switch if we need something really badly (most the time, I get tired of being the medic after an hour or so).
Overall I'm best at Heavy, but I love it when I get in the zone as sniper, where every shot just seems to find the head! Also being (made to look like) a good spy is epic, and a bit of a buzz. That only happens against bad teams mind, but single handedly stopping an attack on a CP is a LOT of fun.