The Eye Shadow Shading Brush from Sisley is an angled eye brush. A very big angled eye brush. It's not meant for detailed crease work or precise contouring, but the unique shape and full-density of the soft hair have quite a few uses. The short(ish) but hefty handle makes the brush feel like a serious power tool- if you're a fan of Edward Bess Luxurious Eye Brush, this might be a good complementing implement, because of the two additional surfaces it provides.
I find this Sisley brush excellent for a quick but fool-proof application of a sheer or an all-over wash of color. It picks up a lot of of material and deposits all of it on the lid, so I've learned to be careful with dark and very pigmented colors. I often starts with this Sisley brush and then finish with the Edward Bess for blending everything together. I also use the edge sometimes with lighter crease colors or with any eye shadow requiring the windshield wiper technique.
Bottom Line: Not a necessity but very nice to have.
Sisley-Paris Eye Shadow Shading Brush ($55) is available from Bergdorf Goodman, Neiman Marcus and Saks.
Photos by me.