As a Beretta fanboy, I’m sad to see the M9 replaced, but the Sig is a damn good gun. The M9 was a great gun in the 80’s, but there have been improvements since then. Specifically in ergonomics an modularity. The M9 fits great in my hand, but I have freakishly large hands. Of course, I find the M9 a much nicer looking gun, but that doesn’t really matter to the military.
The Sig has been on my list of guns that, if I could justify another full sized 9mm, I would buy. Heck, I might anyways.