I sure have used it!
Six months after I adopted a rescue at one year old.....I thought perfectly healthy.....he had to have juvenile cataract surgery. Most unexpected. Total cost of the surgery plus extensive aftercare was around $3,800. About a year ago , pancreatitis, again not cheap. I have zero deductible and 90% payout. Monthly premium depends on age, zip code etc, etc. but does not increase with age.
He is six now and running into the senior years. Hope he will be ok from how on.
I prefer to limit my liability rather than face unexpected vet bills. To each his/ her own though.