Spy is a very fun class imo. The whole "Spycheck" thing where everyone checks everyone makes Dead Ringer a very useful watch. Some people fall for it, some have no idea where you went. also, sometimes, to get behind a dumb team's lines, backtrack like you're getting health / ammo. Especially useful when your disguise has little health. Only works on pretty dumb teams though. :/
Also, Dead Ringer and Stock Watch are the best watches out there, at least for my playstyle. I can use the CnD, just not as effectively.