The things to be careful of are the same things as human health insurance, except it's far less regulated. They will have standard ways conditions are treated, and you'll have to get whatever your veterinarian wants to do approved.
They often don't cover genetic or congenital problems. Does your golden retreiver get ear infections? That's common for golden retrivers, it may not be covered. Does your german shepard have hip dysplasia? That's definitly a genetic condition, and there's nothing they'll do to help.
There are two ways for insurance companies to make money - chargeing more than the costs of what they provide and negotiating low costs for the same work. I havn't heard of any pet insurance companies doing the later, so it's pretty much gauranteed that on average you will pay more to get the same thing with insurance.
You would come out ahead if your pet has a unprobable catastroiphic ilness that is covered. However, it's likely it won't be covered to the extent that it could be.
Insurance could be great for pet owners, the cost of vet bills are rising to new heights as people insist on the same treatment for their pets as they get in a human medicine, but someone has to pay for it and historically insurance companies have been interested in making money, not spending it.