I originally got certified 8 years ago without a mustache, but I had a helluva time with mask leakage. It took me forever (3 years) to find a mask that fit my head (its not just large but wide). Once I found a Mares Lirica I've stuck with it, even if they are a bit expensive. I've tried an Oceanic Shadow in a shop and that seems a good fit as well, but its just as pricey as the Lirica.
I've tried diving some with a mustache, but since the girlfriend doesn't like it anyway I keep the upper lip pretty clean. What seems to give me more of a problem is 1-2 days of upper lip growth (if I'm on a trip with out the missus).
I do have a chin beard, but as long as I don't mix up my reg and mask that usually doesn't cause a problem.
Only a slighty different subject, 6 years ago just before a trip to Grand Cayman I shaved my head (what little hair was left that is). The day we did Stingray City I lathered my big ol'knoggin with suntan lotion. What a huge mistake, I sweat a lot (profuse only begins to describe it) and by the time I got in the water my eyes were burning as if I'd rub them after making a jalapeno pizza. If I didn't have a video I'd have no idea what Charlie the Moray looks like. It was one of the most uncomfortable dives of my life, and I was only in 4-5 meters of water.