Healthwise, I think it would only be a problem if it was Pure 100% SF6. Then you might have oxygen deprivation. I think it is used in some medical procedures. I'm sure its effects are diffrent on anyone. And although it may be harmless to most, I wouldn't do it on a regular basis. Kinda like the opposite effect that helium has on the vocals...
I remember doing that as a kid, but don't think I would attempt it nowadays.
In Memory of LCPL Ralph Fabbri 9/29/89-9/28/10
Semper Fi, Brother