There are a few reviews on google that are largely positive, but no real long term ones. They're v popular with French and American eventers. They don't seem to be a thing over here yet.
I've got one on order at the moment but the order is on hold due to a manufacturing delay - the company have been good with communication but it's been nearly a month since my order and I still don't have the sleeve or a date for delivery :/.
I was desperate to get one but the heafty price tags & lack of reviews (pos&neg) put me off. Someone suggested to me trying the equilab app and its amazing!
Probably not as high-tech as the Seaver girth but it does everything I need it to do for now. Would still like the Seaver girth at some point but, for now, the equilab app is doing the job fine & its free!
When mine turns up Im happy to lend it to people within a 2hr drive of Swansea! Or potentially a 2hr drive of St. Andrews as I go there for work semi-regularly .