From a sub's poínt of view
I have one (in black leather) and have used it quite a bit, though not for sucking cock in action yet, but I've asked others and tried out a bit how it would work over a cock (check my profile if you wonder...) It's a kind of gag I knew I wanted as soon as I heard of them, they look great and the idea of turning your mouth into a silenced fuckhole is sooo hot.
It's true you can't really move your lips much freely about the shaft, if it's pushed in or held with the tip just outside. I use mine putting the ring between the tooth wall and the lips (it fits that width so there is absolutely no escape once it's buckled tight). The lips then will "cover in" the ring a bit. I'm sure some others have the type as a bit gag, placing it right between the tooth lines; then I think the lips would have less space to move down. The risk of having the tongue pressed against the ring would be higher, too.
The Holder of the sub will have to do the maneuvering, mostly, if He wants a blowjob - and after the cock gets thicker, it's harder to actually move it back and forth, it gets sort of stuck at a certain point vs the ring.
You can't deep throat your sub/slave with a ring gag applied, though that happens in some stories. It would have to be removed a bit into the fuck. Well, we use condoms too, many of us, so why not take that trouble if you want to combine the use of a ring gag and deep oral.