Definitely don't read this book if you haven't read the entire original Mistborn Trilogy. For one thing, it won't make sense, and it will spoil a lot of important scenes throughout. Brandon will give you the whole warning spiel in introduction. Listen to it. My review is intended for people who've already read this and the rest of the Mistborns and just wanted to see what others think. 
 I think this was a lot of fun. It start off a little weak (for Sanderson that means it's still better than a lot of other authors' "good" writing), but got progressively better, and was pushing great when it was over all of a sudden, leaving me wanting to reread the original trilogy. 
 I find it hilariously annoying that Sanderson can pawn off what little bad writing he does as bonus material and leave us thanking him for it. 
 Spoiler warning after this point. 
 This book does fill in a lot of holes, but it brings up quite a few new questions as well. For those of us wanting more Kelsier (I didn't, but after reading this and BoM I do) this book definitely brought us up to speed with what's been going on in his death lately. I'm a little confused by what Sanderson allows him to touch and/or pick up in the cognitive realm. I will say this book improved my low opinion of Hero of Ages, particularly the ending, which is the closest I've come to hating a Sanderson book. I really can't stand disembodied spirit battles. All that crap made a little more sense after reading this. 
 I can hardly wait for the next Wax & Wayne book!