They are basically rebranded AVO shocks (GAZ are suppliers to AVO and a bunch of other specialist British suspension brands). The shocks have been out a few years now, but MX5 takeup has only been relatively recent. I'll give them 3 years before deciding if they overcame the problems that the MX5 shocks AVO had.
One criticism I have heard from a MX5 tuning specialist is that they have a ridiculous amount of bounce/rebound adjustment, slight OTT for road use. The condition of the adjuster might be worth keeping an eye on; seems it could get mashed up quickly with dirt.