I was lucky enough to have a good chat with a couple of K-Max pilots. I know they arent sea sprites but they both use the servo flaps.
As far as maintenance goes I cant really answer that one, but both pilots seemed to like the system. But I beleive that the rotor blades can be tracked by using the flaps.
On the plus side the control loads are greatly reduced so there is no need for hydraulics. The actual control linkages look like something off the R22. The system reduces the complexity of the rotor head and reduces weight.
On the down side the trim was a little slow and they would set the trim for the hover and push against it to go forward.
As far as why nobody else uses it, stuffed if I know. Probably just one of those things where that company only wants to use stuff that they have designed.
Hope this helps a bit.