We went with Trupanion because our Vet had just become associated with VCA animal hospitals, and that's the insurance they push. Our cat was getting old, so we thought some sort of insurance was probably a good idea. Trupanion seems to offer a simple deductible/monthly premium trade-off.
She had some diagnosed conditions going in, mainly hypothyroid. After we got her covered, she was diagnosed with another intestinal condition, which they refused to cover as "pre-existing". The symptoms are similar to intestinal cancer, but the cancer specialist recommended against an invasive test procedure because there is only one treatment regime for cats, cancer or not. In retrospect, it could have been beneficial if the specialist had done something to confirm a new cancer. But Trupanion never approved any of the steroids recommended for her, nor any of the veterinary cat food. When she went into the last emergency vet -- who told us she most likely was going through liver failure -- they didn't cover that either. They may cover the euthanasia, but I doubt it. And if they cover any of it, our deductible eats it all.
So medically-speaking, useless. Financially speaking, the premium represented about 1/3 of her medical/insurance expenses. Everyone I ever talked to has been very sweet and sympathetic. But overall, the policy was worthless.