I now own one. Never really had a serious need – most of the time its a simple matter of looking at a power monitor snapshot. But I was recently burned at a client site; a power conditioning device (unbeknownst to me) corrected phase rotation, so that when the unit was switched to Bypass, the phase rotation (backwards in the building) was backwards on the output of the power conditioner. Several motor driven loads (chiller and compressor) were less than pleased.
For the record, Graingers has several phase rotation meters in stock for $70 – $125