I likes the idea and the shape of this YSL Eye Definer brush more than the actual product. It's an eyeliner brush that's a little wider than the average and has a blunt straight edge (think push eyeliner brushes). It's designated for use with powder eye shadow as a liner and for smudging and smoking out pencil eyeliner.
I find that the YSL Definer brush cooperates better with gel and cream eyeliners than with most powder eye shadows (unless used wet) since it doesn't grab enough product when dry. Used wet it's quite Ok, but I still prefer either a pointed tip or a rounded one. I find this brush highly inconvenient for tightlighning as the tip is too small. As for smudging, it definitely works but my personal preference is for wider and softer smudge brushes (Paula Dorf, Louise Young LY16, NARS #15 are only a few that I like better).
Bottom Line: pass.
YSL Eye Definer Brush ($30) is available at the counters and from yslbeautyus.com.