I keep a card of these in my desk drawer for the first day of my period, and the acetaminophen actually knocks out the cramping within about half an hour. The pyrilamine helps me sleep through the worst of it too, which I didn't expect.
The caffeine in these actually helped with the afternoon slump I usually get on day one, and the acetaminophen took the edge off my cramps within about half an hour. Only downside is the drowsiness from the pyrilamine—fine at night, rough if I take one before work.
The acetaminophen does take the edge off the cramps, but the pyrilamine makes me so drowsy I can't take these during a workday. Fine for nights, not much help when I need to function.