The Stack ST500SR has now been discontinued and you can only buy the ST700 now.

It is only as accurate as the measurements you program into it - you have to measure the circumference of the tyre and tell the Stack this info and it counts the rotations to work out the speed. It is 100% accurate with the correct information (according to Datron timing gear). However, be aware that as the tyre wears from it's maximum tread to minimum, the circumference will reduce and give a slightly higher reading (a smidge over 1% @ 181mph).

Please be aware that the speed sensor ideally needs to be on an undriven wheel.

Ben,
The brightness can be user adjusted .