This device works great if you are working in the 1000+ RPM range.  I found it struggled to give good readings with slower RPMs, especially in the sub 100 RPM range. I would have expected the accuracy to be a little better in the lower range since the product description does state that it works for "0-99, 999 RPM".  However, it did allow me to get in the ballpark of where I was looking for in finding the RPM of the parts I was looking at.  One thing to note with this, if the surface you are attempting to measure is reflective at all, you'll get a lot of bad readings.  You have to find some way to eliminate the reflections so that the only reflective surface it is reading is the tape that you place on it.  This is really to be expected because of the method of measurement, but it seems important to note.  The surface doesn't have to be mirrored, I was having some issues with this on some slightly glossy plastics.  I found that a coating of baby powder often did the trick, but this might not work on all parts or with very high RPMs.  Plus, I would hesitate to dirty up some moving components like that.