yes I have this filter, as well as the screw on flat port filter from Mangrove. I think it's actually made by UR PRO, and the quality is perfect in terms of helping to get a good MWB - much better than those of another reputable manufacturer I tried, at least when used with my Pana GH2.
HOWEVER, there is a potential problem with this clip on filter in practice. It's OK at depths below 10m, e.g., but whenever you're in a situation where you have bright ambient light behind you, e.g. shallower than 8m say, you run the risk of getting some reflections from the outer part of the filter on to your lens. Difficult to explain, but the more curved your WWL lens, then the greater the risk of reflections at shallower depths - depending on conditions of course. My bad experience was with the 10 Bar dome, which meant there was a gap around the sides where light could come in, and reflect onto the dome.