Sidi's claim to fame seems to be their heel retention. I'm sure their shoes with the adjustable heel cup system do well to support this, however these shoes lack that system.
Personally I found this shoe's heel cup to be too large (or wide). Down a half size and still didn't ever feel comfortable enough to strap cleats on these things and put em through a test ride; just way too much movement. I don't believe it was an overall size issue, as the rest of the fit was pretty spot on. The heel just did not work for me.
Otherwise, seemed very stiff, beautifully constructed and a fantastic closing mechanism. In the end though, it was Giro Factors for me.