Was thinking and a true damage chain dunk does seem OP, instead of changing the ult to take away from the style of Darius what if once it was used it had a timer similar to Ahris' ult.
If he uses it to successfully kill a champion he would have 5 or however many seconds deemed fair to use it again before it went on CD to use again, killing a second champ would reset the timer etc for 3rd and 4th. He could still chain dunk like normal but it would add an element of strategy to both sides of a conflict.
The defending team seeing that he blew his ult could chose to draw back and wait out the CD before re-engaging with his ult down. Darius himself may have to choose whether or not to blow it early knowing he would probably only get to use it once or twice or choose to wait till the end of the fight and play clean up Master Yi style.
Making this change would IMO go a long ways to balancing Darius while keeping his power more in line with other champions and keep his style of game play in tact.

yeah im sorry i just got killed by non fed darius in 1.74 sec from full health the ult did 948 true dmg which was 52% of my health!! . since when has that been OK RIOT???!

yes it is possible to shut him down. but i believe a ad "bruiser" should not kill that fast or easily. we hat 3 ppl hitting him at that time. so he killed me in the time it took 3 non starved champs to kill him.

Thank you all for the replies but I'm not trying to start a discussion about whether or not he is OP as an over all champion, only about his ultimate.

http://www.youtube.com/watch?v=darop0L43ok