It certainly isn't intuitive, either from looking at the lens or from reading hte instructions. Whatever you do, don't forget the secret. Sometimes the DOF preview can get set without out even knowing. Mine did that recently... I must have inadvertantly set it while putting the lens into or out of the camera bag. I wasn't driven to tears, but dumbfounded by how long it took me to figure out why the viewfinder was so dim. Dohhh.
p.s. I find DOF preview usefull when needed... which isn't often.