The sunny 16 rule does work, and it is based on shooting on a bright sunny day, your aperture should be at f16, and what ever ISO your film is, that should be your shutter speed. For example, if your film is ISO 100 your shutter speed should be 100 or which ever shutter speed is closer. That is the basis of the rule, so based on that you can make an educated guess on your exposure if situations change, like it becomes cloudy or overcast, you can make some adjustments and open up 1 to 2 stops of exposure.
As hongsien mentioned, shooting with transparency film, you do not have the latitude to guesstimate your exposure, whereas with negative film you have more latitude so exposures do not have to be that critical. But if you want to be sure that you get at least one good exposure, bracket your shots(Normal, +1,+2,-1,-2) out of those 5 shots you should get at least one good exposure, or if you want to be even more anal retentive, bracket in1/2 stop intervals. Take this as a guide not the gospel. If you want accurate readings get a light meter, if you can afford one.
Hope this helps!!
Best of luck!