I was looking to replace an old pair of Shimanos and spent a lot of time researching a number of different shoes before I came across Sidi. A lot of the other brands I tried did not have "the fit" I was looking for, but this shoe did. I know sizing among brands can be very confusing since a size in one brand may actually be smaller or larger in another for the exact same size!
For reference, I normally wear a 9.5-10 in athletic shoes (43.5-44 in Euro). I have a moderate to low arch. The width of my foot is around the E size; the Sidi website says their regular width shoe will fit a C-E foot width range. I purchased the Wire in a Euro 44 size (US 10, 27cm), regular width. These shoes fit like a glove. A snug fit, almost like slippers, but not tight in any areas in the top, front or sides of my feet. I have enough room to wiggle my toes in the toe box and about 3-4 mm of space from my big toe to the end of the shoe, with no back and forth movement of my foot once in the shoe. The heel cup does a nice job of keeping my heel in place. There is minimal slip even with thin cycling socks and not having the Heel Security System screws tightened up. The comfort step insoles are a big improvement from the insoles on other Sidi models like the Genius 6.6 and Ergo 3 (which I tried on in stores), it provides more padding and better arch support while not being too bulky inside the shoe, very comfortable. The Tecno 3 closure system is great and easy to use. It provides even pressure across all areas of my foot and can be easily adjusted when things get too tight on a ride. I believe it is superior to the buckle and Velcro offerings on other Sidi shoes.
The construction of these shoes is top notch and the pictures online cannot do justice to how nice these shoes look in person. While on the expensive side, I'd say don't even think about the price because these shoes are worth every penny. With most parts being replaceable on these Sidi's, they should last 5+ years.