I've looked into lots of these over the years for our patients. I have yet see one that is worth the money. By the time you consider what they will pay for: usually two cleanings per year, limited x-rays, 80% for filling, 50% for crowns - and take into consideration the annual benefit limit & factor in what your monthly premiums would be, you rarely come out ahead.
Don't forget to take the deductible into account.
Be careful that you understand any periodontal needs do not follow under the two "free" cleanings and that there are limitations on pre-existing conditions.
Dental "insurance" is a misnomer these days. Most plans have a $1000 benefit limit - the same as it was in the 1960's when dental plans were introduced. If your receive a dental plan as a benefit from your employer, that is great. But realize it is just that - a defined set of benefits determined by the contract chosen by your employer.