I have one. I cannot comment on the accuracy as I have not tested it in that way. It is not suitable for leaving in the tank 24/7. What you do is calibrate it like a pH meter in 2 solutions (1 and 10 ppm or 10 and 100 ppm, depending on the range you need), and then you put it into the solution that you need to measure. The whole process takes just a few minutes.
It gave fairly quick and reproducible vales for my tank water (about 10 ppm, which was surprisingly high, but I have not used a kit on it in quite a while). There is no problem telling what the determined value is.
Assuming it is accurate, it would likely be a good choice for someone that cannot read test kits well.
Also, assuming that the standards that come with it or can be purchased separately are made correctly, those standards could be used to test nitrate test kits, if one were so inclined.