I know they are pretty good for boats under 30 feet. A couple of Vegas have them. I know the maker is an ex-marine engineer and is based in Brixham/Paington area. Give him a phone and have a chat. The one I use for my Vega is the Navik which is ideal but cost about £1400 new or £700 second-hand.
If you're talking about the towed generator self-steering/battery charging system of that name, I fitted one to my previous boat, a Westerly 33. I found it frustrating to operate - the steering required constant adjustments and seemed to be particularly sensitive to currents. I pretty well gave up on it after a couple of seasons. I think they stopped production a couple of years ago, which probably tells its own story.
Good, if expensive, for charging the batteries though.