I've recently purchased a pair of black KSO's, I've only worn them a handful of times (currently wearing them for the third time as I type in fact).
I suppose I'm just after a little reassurance that the size I have brought is correct! I'm in the UK and brought mine in from the US as it's proved impossible to find anyone that stocks anything under an M40 here, and I've got small feet!
Anyway. I went with he M38, which is exactly as the Vibram measurement tool suggests. All toes, with the exception of the fourth, are comfortable and appear to have the correct length. The fourth toe however seems to be a little tight around the base. It doesn't cause me any pain, but it's a little distracting and I'm definitely aware of it when out walking.
I've done a bit of reading around on here, and that suggests things will loosen up a little over time, but I've not seen anyone else have an issue with this toe, it seems to normally be the big toe or small toe.
Oh, and I don't intend to go running in them, I've had operations on both shins, but I'm looking to embrace the more minimalist approach to try and strengthen the feet and lower limbs for my gym/weights work.