Have you ever had one of the "bad guys" be an ally in your game? How'd it go?

With Eberron being a bit more murky on who is bad and who isn't, I like presenting my players with the option of siding with a normally "bad guy" or group.

As a DM, did you ever run one of the big bads as an ally to the party? I'm talking about characters like The Lord of Blades, Mordain, Erandis Vol, The Daughters of Sora Kell, etc... How did it turn out for you and how did you get them to be "good" so the players sided with them?