I love my ValentineOne. If you learn how to read it, it is very reliable. I've got it mounted above and to the left of the rear view mirror with an invisicord running to the mirror so it is hot whenever I key on.
Even if you are not speeding, it is a good reminder to check the speed limit whenever it goes off.
I've got a Garmin mounted in the car, and one of the best functions that I like is that it displays the current speed limit. It is pretty accurate, and really helps if you are coming into an area with a speed limit that drops down.