I'm building a new bike. I've always ridden 172.5mm cranks because that's what came on the bikes I bought. I purchased a Quarq from a teammate with 170mm arms. After considering replacing the arms with 172.5s, researching the web for calculators, searching forums including this one, etc., I came to this conclusion:

Nearly everyone is full of hoo-haw, I won't see a noticeable difference, and for the first time in a long time I'm not making another impulse based purchase.

Discuss.

The way I figure it, that's 1.5% difference in overall pedal travel top to bottom (2.5mm * 2), assuming the distance is measured from the center of the crank arm to the center of the pedal hole. It also would work out to about 1/10th of an inch difference in crank arm length. I have a bike with 175s and another with 172.5, I can't tell the difference... and I certainly wouldn't spend the $$ to replace a perfectly good set of cranks. If I was building something new or replacing a set of damaged cranks, I'd buy the appropriate size, of course.

It was exciting getting used to spinning 175s and riding 165s for a while.