I don't have the world's thinnest or thickest brows, but 8 years ago my brows had VOLUME. I got this product though because I want to make them look more volumized. I don't want insta-brows or anything too perfected, I just want my brows to look fuller, even if they look messy because I already have a bit of shape to them. IME, if your brows already have a decent shape or well defined shape, you don't need to do that much to them for them to look good, like this product.
PROS: It looks really natural. Even though it is a fiber product, it looks more like a brow tint. Whatever fibers are present, you can't see them one bit. I also think it's one of the few products that I think look better the more you build it up, instead of looking like makeup or fake. It also is extremely long wearing - I've had this on for well over 24 hours (yes, I slept in it) and it looks exactly the same. No crumble, smear off, nothing. It's not very difficult to remove either. And the brush also doesn't pick up tons of product or need to be wiped off or create clumps or any weirdness. It looks like nothing special, just some design with dinky ass short bristles, but it truly works well.
CONS: The first time I used it, I was ready to return it. The only reason I didn't was because Walmart didn't have the shade down and I wasn't in the mood to go to a ton of stores, so I thought I'd give the product a try and see if it was good, even though the color was off. I have this in dark brown and although I have dark brown hair, it's not black. I have natural highlights in my hair and brow hair too from when you're a child that makes them more dimensional looking without being obvious. This is why I often go for dark brown of softer blacks (if I had to choose black) when using mascara products, they are not stark or harsh looking compared to black. The dark brown in this fiber mascara might as well be black and yes, it comes off as stark or unnatural. The first moment I applied it, it's like "what have I done?" because it appeared kind of "inky" too, like just ink like or not matte though. It dries down a bit less shiny after, but you can kind of tell I'm wearing this, even though it is natural looking in the sense that you can't really directly see it.
OVERALL: This product is really a 4 or 4.5 to me but I'm giving it a 5 because of everything combined. It's cheap, it's easy to use (just short swipes and build it up), it looks natural, and it stays on forever. I'll use this tube up and drop down to the medium brown shade because the dark brown is really a soft-er black is "woah" on me right now, so I'm trying to go in with a lighter hand.