Well, my totally non-scientific opinion is that with my short legs, there is a huge difference in feeling between 165 and 175. I basically cannot ride 175mm cranks comfortably. I have to move the saddle down about 3cm to make it somewhat comfortable--I don't know why, and really, I guess it doesn't matter--but this makes me strongly prefer 165mm cranks. 170 I can tolerate, but I will always choose 165 if available, and if it's not available I will think long and hard about using a different crankset.