I haven't used it on Macs in years, but on PCs at least, it is a very, very, very, very, very bad, bad, bad, bad, bad thing. NUM 3.x used to be good back in the days of System 7...Norton Disk Doctor 3.1 saved my ass too many times to count back in the days when the LCIII was my only machine. But in modern times, on PCs i find it causes more problems than it helps.