I love these shoes! Running with these on a hot summer day is an amazing feeling; the air literally blows right through them making your feet feel cool and as if the shoes ain't there. I'm still using them now in the fall, will see if winter works too :)
The downside though is that they don't handle what they're made for very well, that is in water. As soon as you step in a lake or a river, small particles of sand flows into the shoe -I'm guessing through the big draining holes in the front- which makes you have to take them off and flush them out and wash your feet before you can continue your run on land. However once they're out of the water they feel great running in and they dry quickly. I often ran down to the lake this summer and just dove right in with clothes, shoes and everything! (I'm a big fan of polyester clothing :) )
Oh, and these are also great in the gym playing sport, I don't think they leave any black marks, at least I haven't seen any, and the traction is great, not to mention the feeling of control when your feet is just a few mm's from the floor! Just don't wear any polyester socks or similar, use cotton or maybe wool, otherwise your feet will glide in the shoes when you make quick turns and you can burn yourself or get blisters.
I usually have size 43 EU when it comes to shoes from other brands, but for these and Merrell's other barefoot shoes I need 43.5 EU.