Been using it on my hobby cnc for over 2 years with never a problem with Mach3 itself. I did have one PC that caused Mach3 to do some extended moves once started, but I tracked it to a rouge .dll that had nothing to do with Mach3. (other than to make it act strange)
After that I did a clean install of XP and only enable what I needed to run Mach3 in XP and BIOS. Have never had that problem since.