I tried making one recently using a sabertooth and a picaxe. Easy to do but the update rate is to slow for a quick servo. I was using a drill motor and gearbox as the output with a ten turn potentiometer. So to the picaxe it should have looked like a ~90rpm servo (~0.1sec/60degrees) but it was too fast and the ADC resolution too low - it kept oscillating unless I made it slow down a lot. I've got plans to try again with a faster microprocessor which should be no more difficult.