If your desire is to make your necks perfectly convex, then this is a tool for you. But, if your intentions about using this tool are more on the traditional side, then you might find yourself to be itsy-bitsy, teensy-weensy disappointed (at least if you decide to check if the tool is actual straight or not; it's always possible that my particular item happened to be a dud, but I do wonder how many people who bought it actually went to check the straightness of the tool and how many of them are setting their necks perfectly convex or concave, enjoying the comfort of the blissful ignorance).
The straightedge I got is convex on the long scale side. I've checked it on various surfaces and I also did the comparison of the long scale side with the short scale side. The short scale side just gave different readings; I've thought maybe the whole ruler is somehow bent a little, like a banana, in that case the short scale side would be concave, but the comparison didn't seem to suggest that. The short scale side just gave noticeably different measurements, suggesting it's not bent, just imprecise. The ruler is bent a little when it comes to larger flat side though; something you can check by placing it on a piece of paper on a flat surface, first one side then the other, and checking if you can move the piece of paper beneath it. Maybe that slight bend on the wide flat side of the straightedge results in the narrow side to be imprecise?
Anyhow, precision is a key here, so needless to say, this straightedge would result with imprecise measurements, which kinda defeats the whole purpose of this tool. The straightedge itself is quite heavy and sturdy, it's very hard to bend. Although it's not impossible, I do find it unlikely that this bent of the wider side and unevenness of the narrow side happened post-production, there is probably some inconsistency or imprecision when it comes to manufacture of the tool itself.