Just thought I'd add to this since I got a chance to try Dylan's NEOS and down booties this weekend. I usually wear a 10.5 and can just barely fit into his shoes and booties (which is good, because my feet were frozen and that was the only thing that finally thawed them out).
I have size 11 Salomon trail runners (sized a tad bigger to accomodate extra socks) and tried putting the size L NEOS on them. They definitely won't fit. Without just sucks, the size L just barely fits my foot, with my toe and heel just about touching the ends of the insole. If you're a 10.5 or 11 I'd definitely go with XL NEOS.
Glad to see some info on NEOS. I'm getting some for Christmas, and now I'm glad I ordered a size larger. I was right on the line between sizes & their product rep told me to go up in size. I will primarily be using them as commuter booties over my work shoes so my feet will stay dry & warm this winter. I'm looking forward to trying them in camp over down booties.