It's been years since I looked in to them, but in 2010ish I went with PetPlan...it did get more expensive every (6?) month renewal, but in the end it more than paid for itself. I had a golden retriever who ended up w/ cancer when she was 4, and having the insurance let me not worry about the cost when she was staying overnight in the emergency vet/having scans etc done. In the end she didn't make it, but I would have felt horrible if I at least didn't try every reasonable option due to the substantial cost. I think they reimbursed around $8k and I had nothing but a positive experience with them. They didn't reject any part of the claim or ask for more proof/justification, and paid quickly.
That being said...I was probably a couple years from the point of cancelling due to the ever increasing cost - I don't know if it's really worth it for older pets, but if/when I get another puppy I'm sure I'll get insurance for at least a few years.