Oh yeah have done it two or three times (different girlfriends). Once, my girlfriend came over while a buddy and I were watching some porn. She came right out and said she had always wanted to give head to two gymrats at the same time. She did and then we both fucked her, one immediately after the other. Didn't have any problem with it. We broke up a few months later, but not because of that.
Then another girlfriend and I were in the act in my den once when another buddy dropped by unexpectedly to pick up some tools he had left at my house. She went to the door half-dressed and to my surprise pulled him in the house and brought him down into the den.
We laughed and talked for a few minutes and then she asked me if I'd have a problem with her blowing him. The position we finally landed on that, to this day, I have an incredibly hot memory of was her lying on her back, me pumping her, and him on his knees, down by her head, facing me and pushing his stiff down into her throat and deep throating her, while her eyes sort of rolled back into her head. Just like a fuckin' porn flick.
If you know the guy is a good guy, no problem. I wouldn't want to make it a habit. But it can definitely be hot.