It is especially useful to use it right before changing government out of Despotism, usually for Temples or Libraries (esp. if scientific). This takes advantage of the otherwise wasted period of anarchy. During those 6-7 wasted turns, nothing accumulates except time, which benefits the rusher. The Temple or Library is now that much older and closer to the 1000 year doubling (7 turns in the late BCs is 140 years, or 1/7th of the way there). Unrest or unhappiness doesn't really matter, since you are not producing anything anyway. And one third of your 20 turns penalty ticks off, when nothing would have happened anyway. Note, however, that culture also does not accumulate during Anarchy, so your borders won't expand until afterwards. If that is an issue, rush them 4-5 turns before you revolt, so you get the borders kicked out first.