I'm back to in-order reading of the Discworld novels, and this is another of my non-favourites. Since it's a Discworld novel, there's still plenty going for it - good jokes and good characters abound. My problems are more at the structural level.
At the highest level, I feel too much happens and is then undone. This is remarkably unsatisfying. Beneath that, the plot still focuses on trying to bring a system of magic to the Discworld, and make it its own place. A sourcerer being an eighth son of an eighth son of an eighth son is all well and good, but it's still pushing the world towards a generic fantasy pastiche, rather than the entertaining and thought-provoking mirror to this world it becomes in later novels. It's another Rincewind novel, again providing this unfortunate feeling that he's the star, rather than the world.
I think if Terry wrote this novel after a few of the other books, he could have made it work a lot better. He'd have improved the balance, and have had the confidence and well-constructed world to really pull this off. As it is, it just underwhelms.