Hi, I used to use them all the time when they first came out ( early 80's I believe). As long as you matched the board to the current requirement of your motor they worked well & were a cheap alternative to the available esc's. They are inefficient, as when running at low speed they dissipate the excess power through resistance, so the overall current drain from the batteries is about the same as running at full speed. However at that time most esc's were also quite inefficient, as the transistors they used in the output stage used up a lot of the power, the esc's were also expensive. My models were all small at that time ( under 2 foot) so the boards worked well on the smaller motors, however the boards required regular cleaning & the wiper arms eventually wore through the board tracks. That said they were still a very good, cost effective, way of controlling your boat.
Modern esc's have moved in leaps & bounds in there performance, efficiency & controllability, & by the time you add on the price of a servo, I don't consider the boards worth bothering with these days. That said I still have 2 models fitted with boards that must be over 20 years old, but when they die i will replace them with cheap esc's.
Cheers Glenn
