Not op, but it's not bad but it's definitely not a part of the main three books. You kinda have to consider it like the "jax and the broken house" story. Narrow Road is a bit longer than that story, but it's an order of magnitude less than either of the two 'main' books.
If you like his writing and you're hungry for more of his work, it's a good way to scratch that itch... But it functioned like an appetizer for me, I was sated for a bit but I can tell I'm going to end up even hungrier for the next proper book than I was before.
I would add, as someone who read "The Lightning Tree" but not recently, it is recognizable as the same story with quite a bit added. It's a nice novella and I like Rothfuss's turn of phrase, but it doesn't have the meat of the main story or the whimsy of "Slow Regard of Silent Things"
Also it is a little weird that the author's note starts with "You might not want to buy this book", of course I got it out of the library so I guess I can't complain.
Honestly? It feels like a cash grab. I got it from the library because I had suspicions. The meat of the sorry has been published before in The Lightning Tree. While it was decent and the additions helped round out the story. It's short, not really new, and nothing like the masterful work that is Slow Regard of Silent Things. It's no where near worth the price the publisher is asking for it. I probably won't purchase it like I have all the rest. I'm super disappointed in Rothfus currently. I know he's struggling to write/re-write/re-word/re-imaging the final book and I can imagine how tough that is. But he's used repeatedly undelivered promises related to the final book to push his charity and get questionable revenue from those initiatives. I want to still have hope in him and his integrity, but it's waning. Just my 2 cents.