This is another one of my favorite recent epic fantasy reads. There are so many things that make it great. First and foremost is the way McClellan integrates gunpowder and magic into a magic system you've never seen before. On top of that, there's a ton of action and generally fast pacing. It took me a while to become attached to the characters, but once I did, I was all in.
Promise of Blood is set in a world with an 18th or 19th century feel. There has just been a French Revolution style conflict, with Field Marshal Tamas killing the corrupt king. At the beginning of the novel, we're dealing with the aftermath of this coup, which is a nice change of pace in fantasy. So often you see the revolution itself, and then the author acts like everything will be just fine, and everybody lives happily ever after. That is not the case here.
Throw in some conflict between nations and gods and you've got the recipe for a great story. McClellan manages it all very well. It's not a perfect book, but it's a great read.
You can find my review of the second book in the series, The Crimson Campaign, here.