In my experience, mixed. They will pay up for cases which are pretty clear cut, for example serious injury of horse. However don't expect to ever speak to the same person twice on the phone, and expect each of their employees to give you different advice on how to file your claim. However be aware that if you have a less clear cut case without a definitive diagnosis they may wriggle out of paying by declaring it to be behavioural, with no evidence that it is, and leaving you with a big vets bill to pay.
I have been with them for years and have never had an issue. Have only made one claim with them and that was a few years ago and my mare had a few issues going on caused by a reaction she had and they were fab and paid out for everything with no problems at all.
We have always used them for our horses. Only problem I had was that when Dylan was PTS they initially refused to pay out because we hadn't had a post mortem done (he had an open hind leg fracture - pretty clear what was wrong with him), but our vet wrote them a letter explaining the situation and they paid out then. When my sister's horse died (at the vet hospital), my dad called them to see if they required a post mortem before we made disposal arrangements, which avoided any issues.