I'm using the Rock n Ruler strictly to measure pickup height--I use other tools for other measurements. I also have many electric guitars, so its getting a fair amount of use. I set most of my pickups to around the 2.5mm mark.
Given the above, after a couple of months of use the black notches surrounding and including the 2.5mm mark have already worn out, due to the unavoidable contact and friction with guitar strings when taking a measurement.
This has made it difficult to get an exact and accurate reading with the ruler alone.
While I don't find you *need* to be extremely accurate with pickup height per se, and its best to go by ear, the ruler has as consequence become a less than ideal tool for the purpose of accurate note and reference taking, and needs to be supplemented by a second reading with another tool for accuracy's sake. I'm using a feeler gauge alongside it for this purpose.
This is the only guitar ruler I've ever used, so I don't know how it compares to others on the market or if it's just in the nature of a metal rule to suffer this fate.
If you prefer to take very accurate readings as I do, make sure you have a second tool to hand to check your readings and keep them consistent.