I have VPI and they are pretty good. The only thing that bugs me about them is that they only cover 1 check-up per year. So when you have a new puppy and they have to get a check-up every 3 weeks with their puppy shots, you have to pay the check-up price yourself.
But from what I've heard...VPI is one of the best out there. They do process claims and send you the refunds very quickly.