I've used several makes of servo vanes; Monitor, Aires, Fleming, and Navik. The Navik although lightweight was the most sensitive in light air, the newer Fleming a close second. The servos all steer really well when the winds pick up. An interesting issue Ive noticed is in heavy winds, the water paddle swings up with such force they ocassionally hit the frame (Aires, Monitor). On the Flemings and other vanes that water paddles that swing up 180 degrees, the paddle can potentially swing out of the water and skip on the surface until they resubmerge. This can cause a few seconds lost steerage. . probably no big deal. .Probably the most significant reasons to be careful with this type of gear is that they are powerful, and can damage an old steering system that hasn't been inspected or maintained. Ive seen old tillerhead fittings fail and clevis pins shear on steering pulleys. .
I currently have an older Hydrovane on my 9m longkeel. It steers reasonably well but not as good as a servo. The tradeoff is that I have no lines in the cockpit and I have emergency steering. I can also attach my tiller pilot while motoring or in light air which takes much less amperage than my main wheel pilot. In close quarters I can use it to assist my steering in reverse. The semi balanced rudder makes it turn very easily. If I had the need and couldn't afford a store bought vane I would consider making a trimtab auxillary type similar to the Autohelm or the RVG. I can't imagine a long cruise without some sort of self steering device