Yes I have 2 x USB SS and now the Russian Ethernet version (Thou I'm not saying anything on that yet has I've not done testing so please don't ask.)

I have used the SS for 3yrs and it's been great. To get the best immunity from noise causing hanging it needs running from an external 5v supply not the 5V from USB. Other than doing that I've not had any trouble from noise or crashing.

I use it on an old 1ghz celeron Acer laptop without any problems at all.

It made an instant differnce to the machine giving slightly higher speeds and better acceleration with much smoother motion. I have never in all the time using it lost so much has a single step, this is partly down to my running a safe tuning setup but also from the SS being capable of far more than I throw at it.

The way I use Mach is to setup different tuning profiles geared to jog specific types.! IE: for Woods/MDF etc I tune for slightly higher feeds. Aluminium I tune for torque and Mid Accelaration and for 3D/V carving work I tune for high accelration then load what ever profile I feel best for the job.

3D and V/carving are very demanding on the PP because of the high pulse rate needed for accelaration and if your going to have any trouble often it's here where it shows in missed steps or droping/lifting Z axis etc.? . . . .The SS lets allows me high Accelration without worrying about missed steps.

I fully recommend the SS but If buying today I would buy the ESS because it's pretty much immune to the noise issue's that the SS can have in some enviroments (Thou I think thats often the users Electrics bad implimentation more than the SS fault.?)