Very, very rarely. More often I've been accused of sucking at a game and using the easy option that apparently anyone can do to kill enemies. For example, Halo: Custom Edition a few months ago, I played on a map that came with the original game and ended up as the gunner on a rocket launcher equipped Warthog. The driver drove around in circles around one enemy while I shot at him with the rockets. The enemy died, and subsequently started swearing and sending nasty messages accusing me of being crap and only being able to use cheap tactics to play. Never mind that I'd used other weapons and tactics to kill him mostly, as well as others, in the game and on different maps, plus the fact that using rockets is especially risky close-up and having to continuously adjust your aim when driving means that shooting rockets from a Warthog is actually very difficult and a skilled task. Basically, the guy was a sore loser. He quit after I killed him about five more times for god measure...
On that note, I have often played (mainly on Halo's Blood Gulch map) against people who I swear were using aimbots, due to the fact they were spawn-camping with sniper rifles from the other side of the map and killing me as soon as I spawned, within the second, every time. I never called anyone out on it though, but there were definitely aimbots in use by a lot of people, at least on that map.