Ah, spies. There's nothing quite as satisfying as getting behind enemy lines, either posing as one of them or just being sneaky, learning their secrets, then betraying them in some amazingly awsome way. Preferably with an explosion.
However getting backstabbed by a spy is the most frustrating thing ever. Especially in TF2. The spies, or as I like to call them, complete assholes, is the class that I love to play as, and the class that I hate the most.
Kind of like a real spy. You love how awesome they are, but you hate them when they completely screw you over...
EDIT: Actually, I have an experience from Battlefield 2 I would like to share. The map was Strike at Karkand, I was on the MEC team. I was playing as the spec-ops class and at the beginning of the match I managed to sneak behind the front line, and basically camp at their main spawn. I didn't kill anyone though, I just informed the team commander what was coming. I would tell them what vehicles are on the way, when the enemy was gearing up for a push, ect. It was exhilarating. Sometimes when a tank or jeep would spawn I would sneak up to it, plant some C-4, and wait. Doing that I managed to sabotage any attack they made, and if I needed more C-4 I would have to kill someone, which was the hard part. But I managed to spy on their main spawn throughout the match and because of my actions they didn't even manage to capture a flag.
That was one of the best games of Battlefield 2 I played. And having a team that communicated and worked together definitely helped.
Just goes to show, get a team as large as 32 players working together and its like a well-oiled machine. And the oil is steroids.