Most vets will tell their friends
that it is a waste of money. There are a lot of BBB type complaints of the companies not covering what they should, or in denying coverage based on severity, animals age, etc.
If you are concerned about finances, find a good vet you trust *before* you have a catastrophe. Make sure that they are forthright with you over the pros & cons of any test or treatment. Sometimes the test costs 10X what the treatment does- so it is easier and cheaper to give the treatment if ( clinical signs point in that direction.
Also, if you are worried about not having enough $ in the event of an emergency - start your own fund. Deposit the amount that you would pay for pet insurance into a savings acount every month. It will add up fast, and it will be there as a nice cushion if you need it.