A while back I found them quite terrible, but since then they buffed them to actually smack other units much harder than archers, and as long as you baby sit them, they make great upgraded arties later.
Cities now use the Ranged strength of the siege unit when determining damage... So even if the city can fight back, bring 2 or 3 catapults with a frontline of spears or swords and they won't last very long.
Ever tried to attack 50+


I brought 4 cannons, 2 conquistadors & 2 Terico & still it was so hard to attack that city. There were very few hills so my cannons had to stand adjacent to the city.