I have Pet Plan and love it! I have a limit of $7500 a year, $50 deductible and 10% co-pay. I pay $27 a month each for 2 dogs. They are wonderful and handle claims quickly and don't try to get out of paying them. You can adjust the amount of deductible and co-pay to whatever you are comfortable with.
One nice thing is they cover pre-existing conditions that occur after you get the policy. You really have to read the fine print on those policies, most of them say they cover pre-existing conditions but only until the current policy expires, so if your dog gets diabetes in September and your policy expires in December- after December you will not be able to file claims for the condition. Another thing to watch for is whether they will pay the actual bill (pet plan does) or if they use a list of 'usual and customary' charges which never even comes close to what the vet charges.
I have used the insurance twice, Beck was pierced by a branch and had to have surgery to make sure that it didn't puncture any internal organs. Murphy had two primary canines that didn't come out when his permanent canines came in. Those two incidents would have cost me about $900 if I hadn't had insurance, with insurance I paid around $180 out of pocket.
Here's a link to a site that reviews pet health insurance policies: http://www.petinsurancereview.com/dog.asp You can click on the name of each company and it will link to their website.