A'right... wore the M41 Treks to work today. Very nice. With the leather sole and leather upper, really feels like a glove for your foot. Very nice.
I'm at a desk most of the day, but do make a point to get up and walk around as much as possible, including a few 10 minute walks during the day. The Treks were comfortable all day, didn't feel too big, and actually didn't tighten the strap at all, just closed it loose. With that, almost wondering if an M42 wouldn't be better, but realize at that point it really would be too long for most of the toes.
Got home tonight and put on the M40 Treks that I also ordered. Very tight, more than just comfortably snug. After about an hour, took 'em off and while they weren't hurting, it was a definite relief to get them off.
For the record, my feet measure right at 10 1/4" to the big toes. Measured many times, with different devices, and different positions, always came to 10 1/4".
So for me, its going to be the M41 KSO Treks. I've also been wearing my new M41 'regular' KSOs this evening, they're fitting nicely, I don't see myself going to M40 with them either. By Vibram's sizing, I would be in M40 KSOs.
Adding to the sizing fun is that I'm wearing M41 Classics. So, like many other folks here, including our 'fearless leader' Justin,

, I'm using the same size in both Classic and KSO.
Now, to return the M40 brown Trek, or sell it here... hmmm....