I'm of the same frame of mind as KFdancer in that I tend to avoid all those extra insurance policies that they try to sell you these days!
Also agree that probably only worth considering if your animal is fairly young and in good health. Your vet may be able to recommend the company that they prefer.
When our old girl died, we adopted two kittens here. When I enrolled them with our vet, we were given a month of free insurance. I did actually give that one some serious thought.
One of my British friends told me that what she does instead of insuring her animals, is to have a standing order for roughly the same amount as the insurance would be, paid into one of her own savings accounts. The theory being, the money is then there to pay for treatment if you need it and if not, you have saved a nice little nest egg!
Of course, I decided not to take the insurance and one of the little ones developed a murmur at around 6 months so now I have to fork out for (outrageously expensive) cardiology check ups! It's the chance you take though...do you feel lucky?