Many people appear to find that being gagged exacerbates their sense of helplessness within a BDSM scene, and desire to be gagged for that reason. Gags also have connotations of punishment and control, and thus are often also used as a form of humiliation to the wearer. Some people like the fact that when a gag is used, the wearer has no say in what is about to happen. Some fetishists are sexually aroused by the sound gagged people make when they try to speak.
While using a gag in the mouth might seem difficult or scary, it can really be a lot of fun! Some people enjoy the sensation of not being able to speak, while other people just enjoy the sexy look of their partner being gagged.
When using a mouth gag, the first and most important rule is that you should establish a "safe action." While you both may have discussed a safeword in the past, because you're inhibiting the ability to speak, it's important to adapt your safety notification to be something that the gagged partner can perform.
This is, by far, the most important aspect to choosing a mouth gag. Mouth gags come in various types. They have different looks to them. If you're someone who's going to enjoy the sexy appeal of a ball gag, go for one of those. Have issues keeping your mouth open wide? A bit gag may be more appealing to you. Would you like to be able to tease your partner with the potential of putting something in their mouth while gagged? A spider gag might be your best choice.