So the problem with writing Jane Austen pastiche is that the reader keeps comparing your book to Austen’s books, and that comparison doesn’t go well for you. And so, no, Mary Robinette Kowal’s Shades of Milk and Honey is not as good as Pride and Prejudice. But that leaves a lot of room for a book to still be pretty good, and it does meet that mark.
To the basic Regency formula, Kowal adds magic in the form of “glamour,” which is mostly treated as a decorative art rather than anything of practical utility. For the most part, the glamour doesn’t change the shape of the book too much—if it had just been, say, watercolors, very little of the plot would need to be changed—but it is an interesting bit of flavor that adds freshness to a well-worn formula.
Overall, my assessment of this would be “formulaic, but enjoyable.” It’s nothing special, but it is a pleasant read. I see that this is actually the start of a five book series; given that this was apparently Kowal’s first novel, I can easily believe that the later books would build on that pleasant-enough base with something more interesting.