This YSL Eye Shader is a good pencil/detail brush that can be found at most department stores. It's a small domed natural hair brush that is shaped perfectly for smudging, highlighting the inner corner, applying a thin layer of powder eye shadow to diffuse a too thick/too dark eyeliner and do any precision work one can think of.
I like the YSL brush's firmness and density and its high quality. It's very durable, has been dipped into powders, creams and gels, washed countless of times and still looks like new. It doesn't hurt that this is a pretty and well-designed brush. As such, I don't mind much that it's decidedly more expensive than my favorite pencil brush, the B533BkSL by Hakuhodo (they're quite different, with the Hakuhodo being softer and longer). I do question the $42 price tag of the By Terry Dome 3 brush. A good replacement to the Yves Saint Laurent brush is the Space NK Dome Smudge that has similar size and proportions and performs just as well.
Bottom Line: a classic.
YSL Eye Shader Brush ($30) is available at YSL counters everywhere and from yslbeautyus.com.