Quite an interesting couple of hours today. After a lot of messing about (I don't spend very much time playing so I'm still only level 16), I nicked a helicoptor from Trevors field and then flew, completely unchallenged, into the military base, flew off in an F16 and then spent a while shooting things with missiles in the city.
It's a beautiful thing to fly but targeting other players is impossible. For a start it only wanted to target police. Any ideas how to get it to aim at players?