We published an adventure, called The Giant's Skull, where the players took on the roles of two opposed groups. There were two short adventures, and it was designed so that you played one after the other, rather than switching back and forth.
In the first adventure, the players take on the roles of ogres, who storm a citadel in order to retrieve a powerful artifact (The Giant's Skull) that would allow them to gain dominance in the area.
In the second adventure, the PCs stumble upon the (possibly ruined) citadel and make after the ogres in order to avenge those slain and retrieve the powerful artifact that could give the ogres the edge.
They were sort of designed to be played in either order, but most of our playtesting and feedback had it playing out like that -- ogres first, normal PCs second.
The cool thing about the ones I saw were that the players had such a blast playing the ogres and injecting them with character (and here, I must add a shoutout to Storminator!) that when they later confronted those same ogres as their PCs... well, it was interesting, to say the least. Players that used to wade into a sea of orcs, slaughtering everything in sight (that old "innocent orc" theory was ignored) suddenly found themselves wanting to talk and work out a non-violent solution with ogres rather than killing them! It was cool.
It can be a lot of fun, if you do it right and the players have fun with it.
- James