I sell cars / trucks for a living & own a dealership. I mostly do Diesel trucks & I would say the 6.5 is a "ok" motor - as Towmaster said the Injection pumps are very common & very expensive, I've also seen a fair share of them with blown head gaskets. They didn't have alot of power but people seem to like them for how easy they are to convert to run on biodiesel / vegi-oil. I have had some of these motors with well over 200k miles and they are still tickin'. As with any late 90's Diesel motor they have characteristics - 7.3 Powerstrokes loose high pressure oil pumps & turbo pedestals & oil pans, cummins loose lift pumps as well & blow trannies into pieces.
As with anything they have ups & downs! The duramax is just leaps & bounds better than the 6.5 in every way. Duramax is king!
2005 GMC Duramax CCSB SLT. White / gray, roof, leather. All stock for now.
*SOLD*2004 GMC Sierra 2500 SLE. Regular Cab. Black / Gray, LLY Duramax, EFI LIVE, Kennedy 4 into 5" exhaust, 6k HID's. 20+ MPG tire smoking monster!
*SOLD* 2008 GMC Sierra 3500 SLT - Crew Cab Long Bed SRW. 8" Fabtec Lift, 24" wheels, 37x13.5x24" Toyo open countries, Dynoflow exhaust, Air raid CAI, Bullydog PMT.
*SOLD* 2006 GMC Sierra SLT. 4x4 Crew Shorty LBZ DMAX. Burgundy / Tan Leather
*SOLD* 2004 Chevy Silverado LT. 4x4 Crew Shorty LLY DMAZ. Charcole / Gray Leather