I use slavery through out most of the game until I can pay for rushing...
Most of the time I find it crucial to build military units because the AI will always try to out do you... (yet they're the research leader?!?!?!)
The time spent to build buildings is time better spent doing something else with your city... Not to mention anything you wish to have built won't do you any good sitting in the que...
I don't rush if the city can still grow without issues, but once you see a red face, then it's time to kill off (the more the better) a few heads to get that building built...
On noble epic, it works good, maybe even better on the later levels because they really cap your growth early on...
They say you get a

for 15 turns, but I still see the same # of

and

after I rush... Maybe I'm just not looking close enough... Nor does the

for slavery show up when you hover your mouse over the

count at the top of your city view...
They stack though, and since it don't show you how many turns before you can safely kill off another worker it really hurts sometimes when you go over limit...
floodplains are usually not very productive, so slavery helps to get that city the vital buildings it needs without wasting too much time in between units building...
3 more turns / unit is a lot better than 30 more turns / building...