i'm not opposed to the occasional plank, etc., but if you're training right, this whole ab circuit thing is really superfluous, and should not be necessary. you should be gassed after a strength training session, and this type of thing should stress the CNS too much - if you're training right to begin with.
i've yet to meet anyone good at pull-ups and squats with bad abdominal strength. focus on compound movements and your abs just about take care of themselves.
"dean ornish and dr. davis think the palmitic acid our bodies use for fuel while we sleep is poison if we eat it. zero-carbers like charles washington think the oldest fuel in our evolutionary history – glucose - used by organisms a billion years ago and without which the brains of modern mammals cannot survive for more than a few minutes – is an unnatural toxin if you eat it. both views ignore basic facts of medical physiology and defy evolutionary history." - kurt harris