Harlequins are a stable in the old-skool fluff of 40K; with flashy colors, exotic weaponry, and a penchant for entertaining slaughter, they were the dream army of many of my 40K group members.  After years of stagnation, the Harlequins have finally been updated.  AbusePuppy on 3++.net has written up an excellent review of the new codex, and it is worth checking out.  Without further ado: read the skinny on Codex: Harlequins.