I have some experience with the Sig 12-24.
Pros:
12mm is plenty wide on the 1D (aps-h).
Very, very little distortion, probably thanks to the large bulbous front element.
AF is pretty reliable (though the DOF is very large from the get-go)
Very sharp, in my opinion, from f/8 and narrower at all focal lengths, except maybe 24mm, which, I think is probably the weak point of this zoom.
Relatively cheap, and there´s no feasible alternative to it (except the Nik14-24, and it´s quite a lot more, in addition to the G-EOS adapter).
Cons:
Not weathersealed, but some electrical tape can fix that.
Not a huge aperture, and not a fixed aperture. I think that lens could´ve been made with a fixed f/5.6 aperture and stay as good as it is.
I´ve borrowed this lens from my dad on several occasions, but I´m using a new computer, and none of the shots I did with the lens were really keepers (boring shots, user error...).
Still considering getting one of my own, when funds and the woman allows, for landscapes and street photography. Great lens!