Well, I used it last night and it matched up well with my speedo. 30mph was on the button. I only got up to 40mph, and recorded my average speed. it can also record your distance travelled and time taken for a trip - useful!
It was still visible when in traffic with streetlights and other cars infront of me, but only slightly. It's almost like the reflection you get when you put a piece of paper on your dash in daytime. I think you could see it ok in the day tbh, in most conditions.
It's a bit of fun really, not a serious tool. Got confused once, said I was doing 24mph when stationary, but sorted out soon enough.
I may post pics later if I can.