For what its worth. I have had 2 windmills now for 8 seasons. I too have been very fortunate not to have had any problems with mine. I placed my mills close to the pond with a good slope on the buried air line to minimize the chance of freeze up. I clean my stones every year and replace the check valve at the stones every year. With that stated, if I could have run an airline and used an electric pump, I would not have messed around with the windmills. I am dreading the day when I have to fix one of them with 2' of snow on the ground and minus -10F air temp!
IMO...Windmills have their place but should be a last option and most certainley NOT considered an inexpensive option.
