I am in love with this eyeliner! It is my first UD eyeliner, and I purchased it because I tend to really like the look of cooler toned taupe eyeshadows on my eyes. So when I saw this, I thought, why not try a taupe eyeliner? For reference, I am NW10-15 with brown eyes that have a green ring around the brown.
Mushroom is quite complex and unique. As others have noted, it's a cool toned, medium grey taupe. However, it does have some brown in it as well, as well as very fine shimmer particles. Once it is applied, it has an almost wet-looking sheen. It applies super smoothly, but it does set quickly, so if you want to smudge, do it right away! I apply it over the WnW Fergie primer, so I cannot attest to how it sets and wears without primer, but under these conditions it lasts for nearly all day.
As I am very fair, I find that it is more interesting than brown liner but far less harsh than black, and it seems to enhance the green bits in my eyes that typically get drowned out by the brown. :) I think this would work well on just about anyone, but the types of looks you can do well with it are likely going to vary depending on your skintone. For example, on me it probably looks a bit more dramatic and smokier than it would on someone with a darker skintone, but a darker skintone could wear it well, too, if they aren't going for a dramatic lined look.
I expect to go through it rather quickly as the tip seems pretty soft and will likely need frequent sharpening, but I don't care, because I will gladly repurchase this multiple times.