I have a love-hate relationship with this book. To go against my personality and firstly focus on the positive, Haydon creates a flawless and confident magic system that is a lot more in depth than the standard "genre fantasy" fare of mock occultism.
Also, she's pretty brave in her efforts to make her world truly fantastic, again deviating from the genre fantasy staples of making a medieval world, inserting a few dragons and mages and calling people middle-english names. The plot device of time travel along the root of the world is an interesting one, and a challenging setting to tackle. Her characters too, though slightly irritating at times and not wholly developed, stick in the head just a little.
OK great, now I can be critical; this book grates on my consciousness with it's use of contemporary american language; yuck. The "humour" is occasionally amusing, but only occasionally. The angle character is not always clear, and the plot falls down a few times...it's one of those books that by 3/4 of the way through you can tell is not going to finish itself and will therefore need a sequel; this only works if the book itself was thrilling and not just an obvious lead-in to a long series. Rhapsody gets slightly repetitive in Haydon's attempts to hammer home potential plot strands.
So those are my views, but then again, as I read the whole thing in a short time span and was jealous of the magic system, I must like it, or at least, I can see why this has such a large market.