Well in another thread I had posted that I didnt use them because of the accuracy/cost ...but recently I had 2 full crusader stacks get decimated by them... I was sicily at the time defending rome in one of my unwinnable scens.
In fact they were so effective that I only sallied a single cav to destroy 1 tower...the cannons took care of the rest. The AI actually tried bringing multiple rams to the gate (4) which was amazing...since they usually never do that.

I still dont understand this building though...because Ive had them on other seiges and they worked very poorly... perhaps its the style of city? the placements of some city/faction towers seem better than others... some get a citadel on the corners others do not...maybe random accuracies? do towers shoot better if there are masses of troops next to them as opposed to a 1/2 unit? Hit or miss still for me I guess.