what ring gags are good for
True what was said about ring gags. Everytime I read a story where someone forces oral sex through a ring gag I laugh--'cause whether they are plastic or leather-wrapped steel, the diameter of the ring is such that the whole in the middle is pretty much useless. (unless, of course you're blessed with an extremely small member).
From the m/sub point of view--you can certainly stick your tongue out through it--but it's hard to be very effective with it, and very tiring. More a frustration for the sub unable to serve properly, than pleasure for the domme. (That may not be a bad thing... it becomes a punishment)
However, ring gags mate beautifully with pump up penis gags (speaking from experience, of course)... especially for people who tend to chew through rubber quite quickly.