Having worked on a screenplay adaptation of this story, I've spent a lot of time with it - which tends to make both its good qualities and its flaws stand out more to me. However in my opinion, the books attributes definitely outweigh its issues. Oh, and before you read more... spoiler alert.
The biggest flaw for me is the fact that despite Shai arguing at the start that the project is essentially impossible, we don't really feel her struggling to complete her work on time. While it could definitely be argued that that isn't what the story is really about, I do feel like it still results in a slight lack of forward momentum and tension. I also find that people tend to complain that the primary antagonists - Frava, the Bloodsealer, and Zu - feel rather flat (though I personally love the Bloodsealer), and while the Bloodsealer and Zu do get their showdowns at the end, Frava and her counter-forger unfortunately just sort of fade into the background without ever really becoming true threats.
Those issues all presented interesting challenges when trying to adapt the story. However, one of the advantages of stretching a novella to fill a full 2 hours is that it lets you take time to flesh things out, letting me do things like flesh out the Arbiters and the Emperor a bit and try to add more urgency to the constructing of the Emperor's soulstamp. I also realized early on that the story also lends itself to being told in an interesting non-linear style, which was a blast to write!
As to whether the story is "Good" or "Amazing". I don't know what exactly the qualifiers are for either status, but in my opinion The Emperor's Soul stands as one of Brandon's masterpieces and definitely worthy of standing with the original Mistborn trilogy and TWOK. The deftness with which he weaves its unique themes and magic system are deeply compelling and makes it a great tour de force of what Brandon can do when he's bringing his "A" game. And as a sucker for plot design, I really love how beautiful the story's symmetry is, with lots of powerful moments like Shai's burning of the painting being mirrored by Gaotona's burning of her journal.
So I guess that makes it "Amazing" in my book.