WOW Great topic. Was just looking into this myself. I'm 171cm tall thats what around 5'7" with a 31" measured inseam and it seems that the 172.5 is the correct crank length for my inseam measurement.

I've opted to switch my MTB over to 172.5 cranks as well. Tried 170mmpreviously and they were too short - d=cadence rose noticably with little power. with the 172.5 I seem more comfortable and able to produce power without to much mashing of the pedals.

My LBS also recommended that I have all my bikes with thesame crank length to help prevent injury.

Some good advise, I'm taking it.