I have a seacure mouthpiece, and it's something I would recommend. They are quite rigid, (no chewy-squeaky noises, as I am so inclined, which helps me relax), and comfortable if they are cured right. You have to take the time to mess with them to fit you just right, and to trim the excess tags and other hangy things from them. I had to trim the heck out if mine, because it's a lot longer than a regular rubber mouth piece and will cover some of your molars, and as has been mentioned above, it can certainly trigger a sensitive gag reflex.
I had to take mine off during my last dive trip and trade for a smaller one due to a broken molar (ow!), but put it back on as soon as I got the tooth fixed.
As I recall, it was about 30 bucks at my LDS. Money well-spent. Makes your reg a little easier to identify on a cattle boat, too.