I know i'm coming in very late on this topic but i'm new, so Hi!
I've been with Healthy Pets for about 3 years now and have had no problems whatsoever. They've always been efficient with dealing with enquiries and quick with paying out on claims.
For the price i pay for all mine (approx £30 for about £5000 coverage each) i'm very happy. I have six cats (1 bengal) so unfortunately putting some money away in a savings account isn't really practical. And, as someone has already said, you could have a very large vet bill and you're stuck with putting it on a credit card etc.
One of my cats died from a brain tumour several years back. She went from outwardly fine to passing within 5 days. Throughout that time we had scans and tests etc and it come in total to just under £2500 so personally, i would NEVER not be insured.
We were lucky in that we were with Tesco's Pet insurance then and they just about covered it but i never really liked them very much and i had to chase the claim up several times (although when i finally got through to a supervisor, she was extremely apologetic and efficient and i had the cheque the next day.)
Anyway, i would recommend Healthy Pets. I've considered Pet Plan previously but with 6 cats, it isn't really financially viable right now.
Just thought i'd add my two pennies worth!