Yea. It's just a matter of knowing when and where to use the right class.
I prefer the Spy myself, just because I've been getting better at navigating maps unnoticed. The only problems I have are the usual cramped hallway, which always leads to an embarrassing death after bumping into someone around the corner
Demoman's my other favorite, just because his guns have a little more versatility. You can really rack up an insane amount of kills if you hold a choke point. One of the maps I hardly ever play (was it custom?) had this bridge everyone was trying to cross, and both teams were just piling onto it. I sat behind a rock at a pretty far distance and picked everyone off one by one. I don't think I've ever dominated that many people before :D