The main problem with the 6.5 is that it really is a lighter duty diesel and was introduced against the older Cummins and fords-which have higher torque and hp numbers. Gm decided to get cheap on production with the blocks, and then utilized that stupid electronic pump mounted driver starting in 1994. Also, they undersized the turbo and cooling system. At the time, they were more difficult to work on from a diagnostics standpoint when compared to similar era competitor diesels, which is why they kinda have a bad wrap. That and a bunch of morons determined that if they can't roll coal, the engine must be junk.
I am on my second 6.5, and my brother has had two as well, and they run fine if taken care of and you know the limitations. If maintained properly, they are fairly cheap to work on, unless your crank goes or the block cracks.
Gm finally figured out a lot of the problems and corrected a lot of those on the duramax motors (which have a whole other set of problems).
All that aside, if mine ever gives up, I'll probably 6.0 ls swap it-so many options with that motor.