TL;DR - Accurate if clamped properly. Very tiny and fragile. Hard to use.
This thing is tiny. I mean really tiny, the whole head is about the size of my thumb-nail. It is all made from fragile plastic, including the rod that clips to the violin. Don't put it anywhere that it will get bent!
The readout seems to randomly change from the right way up to upside down. The head is so small that my large hands may be pressing some button when I clamp it on to the fiddle. But to be honest there seems to be no way to do that without pressing something or another as it's so small.
If you get it clamped on right it tunes accurately. But if you don't it will give inaccurate readings. The problem is that it is so light that it's hard to know if it is clamped on tightly and it feels so fragile you don't want to press it very hard.
Pros - Very small and light, easy to put in compartment of fiddle case.
Cons - Need to be very precise about where you place it and how hard you clamp it to get accurate readings. Also very fragile.