I can't comment on buying a wind vane steering, but if you're prepared to make one, there is a link to a pretty comprehensive design on G Atkin's website at http://home.clara.net/gmatkin/design.htm
My Windpilot is excellent. I've had it for about 5 years, use it fairly frequently and have had no problems with it. It steers far more reliably than I can and has a remarkable ability to cope with swells, unlike an electronic instrument. I guess that this characteristic is only available to a pilot that operates with an oar that transfers force to a tiller or wheel.
I've got a Monitor. first on 30' Albin Ballad, Now on Contessa 33. Same gear, new mount. I think it's the dog's whotsists, used for short-handed racing etc. Haven't used other gears though.
I've a NAVIK wind vane fitted to my small sloop and it really is the dog's whatsits keeps a steady course even on run. Bit expensive to buy new though approx £1400 in the catalogue. If bought second hand check for wear in linkages! Well thats my opinion but there will be those who disagree no doubt.