There are three types of basic DC servo sewing machine motors on ebay:  The Artisan brand, several that all look like a Yamata or "Family" brand, and the Reliable 3000 Sewquiet.  I have and use all three. They are not computerized needle positioners.  These motors substitute for a standard clutch motor with no table modifications.  The Artisan motor, IF you get a good sample, makes an industrial sewing machine as easy to control as a household machine.  The behavior of the Reliable is very similar and includes a 6 position speed knob that sets the top speed:  extremely useful because you can make one machine fast or slow by turning the knob.  The one by "Family" acts just like a clutch motor and is no better over all.  It has a "speed" adjustment that helps with control, but it is difficult to adjust, and when you get it so the machine is easy to control, it has no "punching power."   The Reliable I would say is the best bet of the lot:  easy to adjust, lots of torque.  A downer with the Reliable is that  while it comes with two pulleys, both are too big for hauling a heavy machine through heavy material and they are not the standard clutch motor type of pulley.  For slow heavy work, you need a much smaller pulley (I'm using the 50mm that came off my dead Artisan ESM400 and happened to fit) and then it's great. Over all, a good clutch motor, like a Feitsew or a Linko is pretty nice to use but the Artisan and Reliable servo motors are like magic in comparison.  The "Family" things have a mechanical brake with the standard annoyances.  The Artisan and Reliable use electronic braking which is nice because once the machine is stopped the handwheel is easy to turn without doing the half step on the pedal.  The "clutchless" "Family" motor might be OK on a high speed straight stitcher or a serger.  Forget it for leather or upholstery or even more than two layers of denim.  For heavier sewing get the Artisan or get the Reliable if you can get a small pulley.  With only a little practice with the Artisan and Reliable, you can do things like tap the pedal to poke the needle through for a turn instead of using the hand wheel keeping both hands on your work!  All types buzz and "grunt" at times as you work, but are otherwise silent.  You have to check the pilot light or see which switch button is out  to know whether these things are "running."  None turns unless you push the pedal.  One of my three Artisans went bad after just a few hours use in a few months.  Niether the seller nor Artisan would answer my emails.  Like I said, with the Artisan, hope you get a good sample.  A big online dealer has told me he has had NO returns with the Reliable.  Until/unless Reliable turns out not to be reliable, my money is on the Reliable Sewquiet 3000.  And if you are good enough to either go full blast or full stop, might as well stick with a clutch.