Within the first few months of having our Lab he had run into some bushes and got a thorn in his eyeball requiring emergency surgery to save it. That cost my insurance company nearly £2k.
PetPlan, while a tad on the expensive side, covers him for £7k per year for life... It is worth the £35 I pay each month to know that I won't have to chose between my dog's life and a new car/holiday/etc etc
I've got no idea if his isurance premium is going to go up this year because of that claim - so that may be the only downside...
Our dog walker is with PetPlan too and has an English Mastiff which is plagued with problems, their insurance ran out £1k ago - and she just went back to the specialist vet hospital. Without their insurance the cash would have run out £8k ago... I know what I'd prefer.
Some people never have problems - so they'll tell you to save your cash and self insure. Others will have had problems, and will tell you that insurance is worth it. I'm the latter. For me, PetPlan have been great, pay up quickly and without argument and with very little paperwork.