I picked up the three-pack of these layflat notebooks from Ugmonk recently, after seeing them on Twitter a little while ago (I'm not sure if links are allowed here, I'm not advertising them you can go google it lol).
I'm not the biggest Notebook Guy, usually I'll get into little rabbitholes about it every once in a while, but I don't use them as often as I'd like. My current favorite notebook is from The Paper Mind, which rocks, and will be what I'll be comparing these Ugmonk notebooks to.
tl;dr You get what you pay for
The Ugmonk notebooks have a cardstock cover that is probably too thin, so it curves and doesn't close completely flat and after being thrown in the tablet sleeve of my laptop a couple times, is already fraying. I can't speak specifically on the paper quality as I just use a Pilot G2, but it seems fine to me. It's a bright white and doesn't show through very much. I'm not sure how visible it is in the pictures, but it seems there's a very slight warping or waviness to the paper.
The notebook from The Paper Mind closes completely flat thanks to it's cover that isn't actually thicker, but has a material that reminds me of Bicycle playing cards that seems to prevent the warping of plain cardstock. Despite being a year old and being thrown in that same tablet sleeve of my backpack a bunch, there's only minor fraying on the edge. The paper does show through more and is more of a cream than bright white.
When I bought the Ugmonk three-pack, it was around the same price as the Paper Mind, though now it seems the three pack is actually more expensive at $45 compared to the Paper Mind's $38. IMO the one from the Paper Mind basically better in every way except for the price, since it's only a single notebook compared to a three pack. I think I'd give the Ugmonk ones a 3/5 (I'd review it on their site but can't for whatever reason), and kind of wish I just bought a second Paper Mind notebook. I wanted a second, separate notebook specifically for notes related to my coding projects, but now that I have this on I'll probably just use it and maybe give away the other two from the three pack.
I'm not sure if this type of review is typical, or if I left out some major aspects, but if anyone wants any more details or pictures let me know!