RMK is another one of those highly desirable and hard to find Asian cosmetics brands, second only to Suqqu in reputation and (lack of) availability. It is easier to buy online, though, and I started exploring it earlier this year. I started with RMK eyebrow brush (don't worry, it didn't remain alone for long) because I was curious about the shape. Classic eyebrow brushes are angled and have a straight edge. The Paula Dorf one is just one example; I'm sure you have others in your brush holder.
The RMK eyebrow brush is a pretty little thing. It's made of water badger hair and sporting a matte black ferrule. The angle is curved and not straight and the hair slightly tapers towards the edge. It looks like a slightly thinner and oddly asymmetric relative of the Hakuhodo brushes I use to tightline (Kokutan E0189 and K005). The result is a softer look when you fill in your brows. It prevents from even accidentally drawing harsh lines and seems to hold more eyebrow powder and deposit it evenly.
Is there much of a difference between working with RMK's eyebrow brush to a really good traditional angled one (Hakuhodo or Shu Uemura, for example)? There is some difference and I love this brush, but I can't insist that everyone goes out of her (or his) way to buy this brush. If you already have a couple of good eyebrow brushes and you're comfortable and secure in your brow routine than you're probably all set. However, if you're unsatisfied with the look you achieve using your current tools you might want to look into changing your brush shape.
Bottom Line: nice to have.
RMK Eyebrow brush (approx. $29.99) is available at very select locations, mostly in Asia and in the UK (Selfridges, as far as I know). The only Asian site I found that ships to the US is bonboncosmetics.com.