The last model of the 6.5 block was pretty decent and solved most of the problems they had. But it was lack of initiative on GM's part that held it back in the end. Too high of compression, too low of boost, no intercooler, indirect injection... and the list goes on...
With the right set up they can fair decent. But sad to say, yes all the early ones were more often than not, junk. But when they run, they're good pullers, and get pretty decent mileage. Definently not speed kings by any means, but they'll pull.
I saw this a while back, and it suprised me...
TheDieselPage.com - 6.5TD Power & Performance Project