Yeah, Slavery's great. I'll admit it, when I first played I had nothing to do with the civic because of my modern egalitarian snobbery (not that I'm easily offended, of course, I've got thick skin but that doesn't mean I want to actively enslave people). After a few games, I noticed that it had None for an upkeep cost, so I would switch to it since it'd actually reduce my city maintenance. Then I kept losing out on The Oracle by one turn (Ha! Now I barely even bother to built The Oracle... How things have changed

) so I finally bit the bullet and used it. Once the genie was out of the bottle, it never went back--A few turns later, I had angry citizens, saw that the whip cost would be exactly enough and they went off to build the Pyramids for me. Several turns later, I had fine happiness, but people were starving because they were sick, so off they went to build my granary! Now I love Slavery--the civic, the actual practice, still not so much.
I usually go with Representation/Bureaucracy/Slavery/ANYTHING/Theocracy for most of the game. All the economic civics are good, so I'll normally take the one that's a Favored Civic of another Civ. If there isn't a Civ with a Favored Economic Civic, then I'll usually ignore Mercantilism since Castles give a free Trade Route, which defeats the purpose of Mercantilism.
My endgame is Universal Suffrage, Free Speech, Emancipation, ANYTHING, Free Religion. Gotta love those Cottages...