I've sucked a couple of guys through gloryholes on a few occasions, but never "until completion".
I find it a huge turn on, especially when it's been in booths with holes on several walls, so others have been able to watch me give service to a guy through the other holes.

I especially remember one occasion, when I started sucking a cock in a booth that was too dark to see anyhting. That cock was actually so huge, I was totally convinced first that it was fake (a dildo). Imagine my excitement when I realised it was the real deal....