I bought these to replace an older pair of Sidi Genius 5 shoes. Although I had the "mega" width in the Sidi, I found that the standard Mavic fit me just fine, possibly due to the design that wraps the foot rather than pinching (similar to the old 3 buckle Raichle ski boots or the modern Full Tilt). The lining also forms to your foot. I found that for the first 20 miles or so they were a bit tight, but by mile 50 they seemed to be all set.
Power transfer on these shoes feels light years better than my older Sidis, though to be fair the Sidis had much more wear. I'm using these with speedplay pedals, and I think that the thinner sole of these shoes helps put the foot closer to the axle than most other shoes. Due to the curvature of the sole, you'll need to use shims for the speedplay base plate, or if you have the v.2 cleats, use the #6 snap in shims.