I tried the raging barbarians setting once as an experiment. I built a custom map where my civ was fortunate to be walled off with mountains and a decent amount of space to develop. Of course, my lands were peaceful. Had only one barb attack early on but the AI civs were sytematically being slaughtered. 4 bit the dust by 2500 BC and while I wasn't worried about the barbs, I felt I was being robbed of potential trading partners, allies and enemies and lastly, any sort of fun.
Back into worldbuilder I went to bail out a few fallen civs and secure the one struggling to survive. The map outside my territory was a pathetic scene of death and detruction. City ruins and fragmented roads everywhere! Barbs on almost every other square and the biggest AI civ (Russia under Catherine) second to mine had only two cities (I had 12) and was within a few turns of losing the capital. The rest were almost wiped out.
So out of fairness and that I never had to worry about in invasion, all the other civs were resurrected and/or given uber defensive units, tech., prebuilt cities and gold.
Then the barb civ I built in the northern half of the map began to fall to the civs.
Now I felt sympathy towards the barbarians.
It was around 1000 AD and suddenly the barbs acquired all renaissance and industrial era technologies.
After a few turns, each successive turn made that girly screaming sound when cities are captured.
The other civs actually managed to survive and turn back the horde, with thanks to the archers they were given with any helpful promotion I could think of, but I did learn, that I who prefers to the empire bulding aspect to empire destruction, does not want raging barbs anywhere near my lands.