I have a 2003 2500HD CC 4X4 with the 8.1/Ally, 3.73 axle. It's got about 175,000 KM on it with no drive train repairs needed yet. Just normal maintenance. As far as I can tell, it still runs pretty much the same as when it was new. I tow a 5er (over 11,000 lbs) in the mountains of BC every summer and it handles & pulls very well.
Be aware that it likes oil; mine uses about a liter every 5000 KM & has since new. Also, do not be put off by the cold start knock. It's just the nature of the beast. When it's time for a new one, I'd buy another if they still made em.