Hi all, I'm a dentist.
OP, I think you made a good decision. Sorry I didn't see this thread to chime in sooner.
OTC night guards have their place, but you really do "get what you pay for". As a starting point, consider that my lab bill alone for a night guard is far more than any of the OTC prices that have been thrown around here. And IN GENERAL, most of my patients are dissatisfied with the OTC night guards they have tried. That being said, they are super cheap and can work for "temporary" night guards or to see if patients will be able to tolerate a night guard at all. If they can tolerate an OTC night guard, they will likely LOVE a custom one by comparison. There are a few patients who prefer the OTCs, but in my experience, they are a small minority.
As for the value of a night guard, they are absolutely worth it if your teeth show a pattern of clenching or grinding. The bite forces involved in this night-time habit are staggering. Parafuncitonal bite forces dwarf the forces used in chewing even very hard foods. We're talking an order of magnitude here. That's why they produce such significant tooth wear. Rebuilding that tooth wear is, again, orders of magnitude more expensive than preventing it. Heck - even if you prevent one single tooth from fracturing, you just saved yourself a $1000 crown.
As for how long they last... this is a textbook example of YMMV. It all depends on how severe your clenching/grinding habit is. The worst I've ever seen is a classmate in dental school who wore through custom ones about every 6 months. In my own practice, I've got one patient who breaks/wears through them every 1.5 years like clockwork. Then at the other extreme, I've seen patients bring in 15 year old night guards that still have plenty of life left in them. I would consider 5-9 years to be average. Unfortunately, another factor contributing to such discrepancy in longevity is that not all custom night guards are equal. I've been disappointed to see patients occasionally bring in "night guards" from previous dentists that are little more than flimsy retainers. These are fortunately few and far between, but if your "night guard" is less than 1mm thick...
One final word on night guards is that the ones I've been discussing are for bruxism/tooth wear. TMD/joint issues are a whole different ball-game and I refer those patients to a specialist (who charges 4-5x the typical "night guard" rates for his appliances). The TMJ is one of the most complicated joints in the body and creating a positioning appliance that stands a good chance of improving dysfunction in this joint is VERY complicated - often involving special 3D imaging of the jaw joint, TENS unit analysis, doppler... the list goes on. Occasionally, these patients can get lucky and an OTC night guard or bruxism night guard (like I've been talking about above) may coincidentally put their jaw in a position that reduces muscle firing or stabilizes the joint, but it also might put it in a worse position. Without sophisticated analysis, it's really just throwing darts.
Those are my general thoughts on night guards. Always, the best bet is to ask YOUR dentists who can look at YOUR teeth/joint and give advice specific to your needs.