I try not to use slavery, at least, not while the game is still in its early stages. This is mostly because I already have a few cities up by the time I would have population available to spare, and I generally prefer to just chop-rush things I need. I usually have at least 1-2 workers per city, so if something needed to be made fast, I always have a worker or two I can spare for chopping. This allows me to have essentially the same effect as slavery without the population drop, the ensuing unhappiness, or the upkeep costs.
The only time I could see it useful was if you had a city that was absurdly overcrowded, and are in a dire need to raise happiness. Then again, this should really only happen late in the game, and there should be plenty of other civics that should be able to counter this (hereditary rule, environmentalism..)