HAHA!!! HELL YEAH, MAN!!!
Yes, I do use a shot timer, for the last 7or 8 years now. PACT Club Timer II. If I'm shooting by myself, I just clip it into my pocket, or the belt if I'll be moving alot/running. If there's a group of us shooting, it's just easier if there's a "timer," that way you can run multiple people through the course, one after the other, staynear them - and I always angle the pickup towards them, to make sure i get their shots recorded. As the person shooting, you just listen to/wait for your commands, and go when you hear the buzzer.
When there are multiple people shooting at the same time, the shot timer will pick up the other shots nearby. If you ran two people through a 10-shot course of fire, 10 yards apart, the shot timer would probably pick up all 20 shots. It's less sensitive with pistol rounds, but it certainly does pick up nearby rifle fire, everytime.
They're sensitive enough that there would be no reason to have to clip it to the gun - they're fine from the belt, or held nearby the person shooting.