If you have a purebred dog, you must choose an insurance provider that does not exclude heritable conditions. Most insurance providers (VPI included, unless they recently changed their policies) will NOT cover genetic diseases common to the breed.
There may be more, but I know that Trupanion and Petplan (the insurance I use) DO cover genetic conditions in purebreds.
I have had a great experience with Petplan. We have had two claims in two years (one per year). Each claim was paid promptly and without question, and they paid every dime after our deductible. I have also heard great things about Trupanion.
So far for us in the past two years, they have paid us more than we have paid them, so it has been worth it for us.