I have insurance on my one and only cat who does not have any pre-exisiting conditions, Pinkerton. The company is called Trupanion (formerly Vetinsurance). I have very mixed feelings. The prices have gone up quite rapidly in the three years that I have been with them. I have never had to make a claim (because my insured cat is perfectly healthy), so I can't say how well they do in that area. My vet recommended that company, and she says that she has many clients who successfully make claims. The prices vary with age of pet, breed (if applicable), and the latest twist (and reason for the lastest price hike) - your geographic area and price of vet care there. I have a sneaking feeling that they will find some reason not to may me out if I ever make a claim, but I am the suspicious type. You must take your pet in for annual checkups for the insurance to be valid. I believe they have a requirement regarding vaccines as well - not sure about that. They may not cover treatment for a URI, for instance, if the pet is not vaccinated against that (not sure about this). They do not cover any routine care, such as dental cleanings. You will have to have a full physical before you sign up, and be careful of what gets mentioned in the medical history - it may prevent you from being paid out for certain illnesses.
I got insurance for Pinky because I have spent tens of thousands of dollars on vet bills for my unhealthy trio of Bear, Emily, and Teddy. I am heading into retirement (soon, I hope), and would have difficulty absorbing a sudden high bill for something like an ER vet. One big illness involving an ER visit, bloodwork, ultrasounds, ongoing meds, etc could end up justifying what I have paid into it. I just spent more on Emily in the past week than I will pay for Pinky's insurance this year. Of course, as with any insurance, you hope you never need to use it. I know many other prefer to open an account and put the same money they would have paid for insurance into some good investment to use if ever needed. I chose to go the pet insurance route. Just be very careful to read all of the fine print and shop around. And brace yourself for a lot of price increases. There are a lot of exclusions and deductibles as well. They may pay a percentage of a hospital stay, only cover specific aspects of it, etc. Buyer beware.