Scubapro and Atomic used tiny mouthpiece openings until about 10 years ago and while their mouthpieces can be used on other reg, but not the other way around. They have now gone to what is the "normal" size used by current Aqualung, current Atomics, Deep6, HOG, Dive Rite, etc etc.
Apeks uses a slightly larger than normal mouthpiece opening, but the difference is only 1/8" wider and 1/16" taller than the "normal" one and you can use their mouthpieces on any of the above regulators, but make sure you get the zip tie tight. This is the "standard" size on rebreather DSV/BOV's as well.
Poseidon uses one that is another 1/8" bigger in both directions and their mouthpieces can't be used on anything else. Both the Poseidon BOV and DiveSoft DSV/BOV use this size opening.
The Apeks comfobite is a polarizing mouthpiece. Some people will only dive with them *of which I'm one*, and others can't stand them and prefer "normal" mouthpieces with bite tabs. If you absolutely love it, then get one and keep it as a spare. Downside is they are quite expensive compared to normal mouthpieces.
It does appear that the patent finally ran out YAY! and DGX has generic firm comfobite mouthpieces for $8. This is about half the price of what they normally are. This looks to be the apeks one, not the aqualung one which is awesome. The Aqualung one doesn't have the ribs on the bite plates and I always thought they felt funny. I'll be ordering some of those momentarily
anyway to
@Roberto Inzerillo if this is just to keep as a spare in a save a dive kit, just grab one of the $2-$3 cheapies and keep a zip tie with it. It will save the dive. I wouldn't carry a "spare" branded comfobite at $20 because the odds of you biting through it are pretty small
