I have my three 'pets' enrolled, because I know I'd be tempted to spend more than what I consider reasonable on their medical expenses and I don't feel it'd be fair to my family.
That said, I have made a claim...it was fairly simple, you need to have each vet that has seen your dog fax over a copy of your dog's file. You then have to send in the receipts for the claim specifically. It really isn't that bad, it was sort of a pain when I filed one because I'd had Oakley seen by like 3 vets. Once they had everything we got a check within 2 weeks.
We could afford medical expenses for our dogs up to and including some exorbitant things...but the insurance is nice so I don't feel I have to make a decision based on how reasonable the cost of a treatment is. That said I'd never treat a dog for something like cancer b'c they just do not understand why you are making them be in pain and hurting and feeling sick fo r along time. But things like broken legs, etc, the insurance is nice for.