There's a great site here that compares the different policies and has customer reviews:
http://www.petinsurancereview.com/
You definitely want to read the small print. As chico2 pointed out, pre-existing conditions aren't covered, and some insurance companies try to find a way to link every other ailment back to that pre-existing condition so that those aren't covered either. But there are some good policies out there, and if I were to get a new pet some day, it would be something I'd seriously consider (after having spent 10s of thousands of dollars over the years on 2 cats
).