First I'll say that I love the Mistborn series and they (after Elantris) cemented Sanderson as my favourite author, so it goes without saying that I enjoy everything that man wishes to turn out.
The Alloy of Law isn't really a mistborn book though--it's more like a Alternate-universe fanfic with a heavy American West spin. I really don't think the Mistborn world would've turned out the way it did in this spin-off. It also felt more like a daytime mini-series where you chuckle at the heroes and roll your eyes at the villains.
I liked it, but for the series this is meant to continue I was rather disappointed.