I use the sunny 16 rule (or an adjusted version if it's not sunny) when I don't have a meter with me (infrequent) or I need to get a shot quickly (semi frequent). I have found it to give perfectly useable exposures with black and white negative film. In an ideal situation I would use my incident meter or a spot meter but sometimes that's not always possible. So I find sunny 16 to be very helpful. One other trick I don't think has been mentioned makes it a little easier to calculate "sunny 16" quickly with larger apertures. Add a zero to the film speed at F5.6, ie 100ISO film is 1/1000, 400ISO is 1/4000.