This fiendish-looking devices is actually used by dentists and oral surgeons to keep mouths open during examinations and surgeries. However, fetishists, medical instrument collectors and Bondage and Discipline fans often find them interesting for other reasons.
This item was designed as medical/dental devices. It cannot be expected to function as secure gags (although, depending on the size and shape of an individual's jaw and teeth, it might be effective in some cases). It is generally possible to push them out of the mouth with a bit of effort.
However, by adding some sort of strap around the back of the head to tie them in place (such as a leather thong, sold separately), it is possible to make them somewhat more secure.
Many people will want to own these simply as a collector's pieces. However, others will want to use them. Obviously, caution is necessary with these items. You could risk chipping teeth against the hard metal if some kind of impact occurs, and it is important not to strain the jaw muscles by opening the jaw too much or for too long. Use care and common sense, and proceed slowly.