The Well of Ascension by Brandon Sanderson
This is the sequel to Mistborn: The Final Empire. It's the second in a trilogy. So...what can I really say? No matter how good the second in a trilogy is, it's never as satisfying as the first and the third! That's just the nature of trilogies.
Still, Sanderson did a great job. Jacob says that Sanderson is the best at crazy twists that you never saw coming but that work perfectly. I can see where he's coming from on that -- there have certainly been some jaw-droppers.
Anyway, happily, I'm on to the third book now. :D