Trupanion is great and will work directly with most vets so you don't have to deal with reimbursment, but it's expensive. From what I've read of petsbest (reddit this), you'll get a bunch of hassle and denials.
If you're expecting big bills soon, it may be worth it. If you're not going to put your older pets through major surgery/treatments, you're probably better off "self-insuring" and just setting the $500/month aside in savings earmarked for vet bills.
Most insurance won't cover exam fees or well visits, and some don't cover standard preventive care (heartworm meds, flea/tick, etc.). Read the fine print.