These are not actually dental insurance, they are "dental discount plans." Supposedly, by joining the plan, some (not all) dentists will charge you less for services than they would if you are not in the plan.
If you already have a dentist you trust, you should see if your dentist offers discounts for members of any of the plans. Then before joining you might want to call the dentist's office to talk to one of the office staff and ask them if the plan really does make a differemce. Also ask them if they offer a discount for immediate payment (you bring a check and pay the same day) since that can save you up to 5% with no plan needed.
If you don't have a dentist, you might find the dentist first instead of locking yourself into whatever dentist the plan will let you use.