Q. Is it easy to use
A. Yes
Q. Does it need checking (calibrating)?
A. Yes - because the readings might 'drift' over the months
Q. If the meter shows the stone might be one of half dozen different stones, how do you know which one it might be?
A. You don't
Q. If the stone is one of several hundred stones not listed on the scale, how do you know what it is?
A. You don't
Q. Is there a machine that is guaranteed 100% reliable, costs less than a few thousand pounds and doesn't require a thorough knowledge of gemmology and physics?
A. No - no such machine exists.
CONCLUSION
This is by far the best machine for giving an approximate identification. More usefully, it often tells you what the stone cannot POSSIBLY be. So, for instance, you might be uncertain as to whether the stone is glass, garnet or tourmaline, but you will know, for certain, that it can't be Spinel, topaz, ruby or sapphire.