Variac works quite well in my experience with my Gyrodec. It's a fairly serious bang for the buck as far as I'm concerned. No ability to control speed (as far as switching from 33 to 45 RPM) and it lacks the elegance and automation of supplies like the SDS but certainly no drawbacks in terms of holding actual speed.
Granted, the SDS probably does more (and should at the price) but HW is actually on record as recommending the use of a variac with certain AC Synchronous motors and the fact is that the variac does accomplish some of what power supplies like the SDS and Lingo do, namely reducing the voltage and, as a result considerably reducing motor noise/vibration resulting in a cleaned up presentation and lower noise floor.
Granted, the SDS probably does more (and should at the price) but HW is actually on record as recommending the use of a variac with certain AC Synchronous motors and the fact is that the variac does accomplish some of what power supplies like the SDS and Lingo do, namely reducing the voltage and, as a result considerably reducing motor noise/vibration resulting in a cleaned up presentation and lower noise floor.