I have M&S insurance for my two younger Newfs and for Tia. I have to say *touch wood* we haven't had any problems with them paying out, even when Kitten had what could have been classed as a hereditary problem. He had to have an op for elbow dysplacia, the x-rays, op and the medication he was on for 4-5 months cost over £1,000 and they paid it back no problems.
We also have the no excess policy, so eveytime we go to the vets we get our money back... Apart for things like their inocculations of course.
The one thing we did find odd was the different costs for the dogs as things have changed since we originally insured the Newfs. When we insured them we were told it would be £11 something for the first one, and then £10 something for the second, as they give a discount for more dogs. We are still paying that now, over a year later. When we phoned about insuring Tia they said with the discount for another dog and the no excess policy it was £17 something a month. It just seems daft that it is so much cheaper for such a bigger dog, I know it wouldn't be now if we were to insure them though, as they have changed things, but we are still paying what we did in the first year.