I’ve driven the DD13 in a Cascadia for over five years (I’m now in a Pete with a similar size and output engine, the MX13).
After spending two decades driving Cummins before that, I have been more than happy with the 13l motors. I’m doing up to 105,500 now with a 13l, and been impressed with the pulling power. I’ve pulled 96,000 up Cabbage and been able to stay at about 35 mph all the way up. Seems pretty decent to me. They’re reliable motors, but if they need repairs or maintenance they can be a little more spendy, from what I’ve been told (I’m a company driver, I don’t have to pay the bill).