I read it only if there is something that makes no sense whatsoever.
Most the time I can find out all the controlls within a couple of seconds. Although I have quite a bit of logic on my side and, unlike many people I know, don't have to waste time asking somebody who has already played the game what to do next.
For example the Splinter Cell games. I see things many people seem to overlook, such as vents, rappel points, pipes, etc..., etc...