I have no idea what the new Codex rules are but back in the day they were absolutely death-on-legs to pretty much anything
They're still pretty good, but for a while they released an online codex just for Harlequins, and (while small), they were seriously incredible
Now they're lumped in with Eldar as an Elites choice just for some really fast moving close combat choices Good still, just not as unique/awesome as they had them in the specific codex
Still some of the most awesome models ever though, I love em.