Maybe it's because I came to Shadowheart directly from its dull predecessor, Shadowrise, but I found this book slow to start. Happily, after a hundred or so pages, the pace picks up as things start happening. Sadly, it bogs down again in the last hundred pages.
Williams brings together a lot of the loose ends in this book, in a moderately satisfying way. However, there is also frustration - for example when Chert Blue Quartz comes up with a bright idea to save the day, but we aren't told what it is. Since the idea is a fairly obvious one, it quickly becomes irritating to have the text dance constantly around it - for hundreds of pages. Williams does make some interesting choices with Barrick, but much of the ending is fairly predictable.
As Williams' first series drew heavily on Tolkien-style epics, the end of this series draws on Greek mythology, with a little Christianity thrown in. To my mind, it added little but complication. There's a lot to be done with surprise and disappointment about the nature of the gods, but Williams, after approaching the idea, backs away.
Much more disappointing was the treatment of the Qar. After being fairly flat in Shadowmarch and Shadowplay, the Qar finally made a genuine showing in Shadowrise. I had great hopes for them in Shadowheart. Williams set them up as a great tragic race, had me just waiting to have my heart broken, ... and then sort of wandered on to other things. He created a great opportunity and then made the least of it. I wish I could say that he simply implied the tragedy with some subtlety, but I don't think that's it, or if it is, it didn't work.
I give the book three stars because the action is much improved from book 3, and because of the potential of the Qar. Nonetheless, I continue to think that had books 3 and 4 our been condensed into one, the series as a whole would have been much stronger. Overall, the series comes out decent, but because of the slow finish (in books 3 and 4, and the end of book 4), it's hard to recommend. I'd say stick with Otherland and go no further.