I had a pair of D4-260's; the little brother of the D-6 for 7 years/2900 hours. Wonderful little engines; they still run like tops; no leaks, no smoke and you can have a normal conversation at cruise.
Things to watch for; ie engine maitenance 101 - aftercoolers need to be maintained or corrosion will bridge the end cap/housing; I found 3 years was the right time to pull them apart and service them. The belt tensioner pulley is a wear part - it really sucks when the bearing goes 90 miles offshore. The computers ( 3 or 4 per engine) were reliable for me but I have heard stories of the computrer deciding to put the engine into fail safe mode and not go above 1200 RPM.
The engine has tons of mid range torque - the 2 little D4's put a 17K Carolina Classic up on plane like they were ooutboards.
Parts are expensive, buit as I just learned - so are Cummins parts!