Updated review.

I first bought this shoe in my usual athletic shoe size 10, which usually provides ample room for thicker hiking/cold weather socks. Upon receiving them and trying them on, I found my 2nd and 3rd toes on one foot to bump with the seams, so I immediately put them on exchange. My intended use for the shoe is not just "barefoot" running, but also some lightweight barefoot hiking and backpacking, so I needed them to have some extra room for thicker socks like wool and goretex. My feet also swell quite a bit after 5+ hours of hiking, and I'm tired of nursing sore cramped toes in stiff boots.

Upon receiving the new shoe, a size 10.5, I became a little concerned that they were just a bit too big, so much so that I questioned if I had done the right thing in the exchange. I haven't worn them for extended periods of hiking yet, but I've worn them being on my feet all day, and they've taken me trail running and jogging in a variety of conditions. I was concerned I'd be unable to wear these without socks in warmer temperatures, but I've found them to be quite comfortable even with the extra room, and I have not experienced any unwanted rubbing. Perhaps because they're so lightweight that even with the roominess, the foot isn't fighting with a heavy sole or rigid exterior to feel like they're "floating" in the larger size. I've read other reviews that mention that they pinch when feet swell, and it was that effect that I was looking to avoid by going larger.

Also, even in their roominess, they're still one of the most flattering athletic shoes I've worn, and I wear them casually with jeans day-to-day. They make my feet look smaller than most other shoes, and then yet, I avoid all the comments and questions that Five Fingers inspire. I only wish they came in dark brown and other earth tones to go a bit better with my overall wardrobe.

So far, on the light hiking I've done (wearing with a pair of mid-weight wool socks), I'm impressed with the grip on dusty trails, and the fabric seems like it'll hold up really well for regular hiking and backpacking. The sole is a bit thicker than most "barefoot" shoes, but with that comes added protection for use on trails that I miss when I'm in barefoot sandals and Five Fingers. The sole also provides extra insulation on cold earth than thinner Five Fingers. There's also a natural, built in lean for mid/front foot landings.

I have a history with Merrell of experiencing shoes sized awkwardly, and it seems this isn't too uncommon. I have a pair of Merrell hiking boots that are a size 10 but are so narrow that at the end of a day of hiking, my toes feel like they're piled up on one another. My partner just purchased a pair of Merrell hiking boots in a whole shoe sizer larger than he normally wears.