check out www.62-65-dieselpage.com - it's a chevy diesel webpage. I test drove one (w/auto) with a buddy and it's a nice truck - drove almost like a gasser compared to my Dodge Cummins w/6spd stick. I'd wait a year or two for the bugs to get worked out.
I've got a 1.5 month old '01 Chevy Silverado LT 2500HD CC SB 4x4 with the Duramax and Allison 5 speed auto. I just turned it over to 2000 miles and so far I love it. It really rips and, even with a turbo, it's got great low end pull. It develops its 520ft lbs of torque at 1800rpm. Reline is 3250 which took some getting used to. But the 5 speed makes the most of it. I had a 99 GMC 3/4 SLT 'burb 4x4 454 before the Duramax. The 01 rides a bit rougher than the 'burb but that's about the only drawback so far.
Much better mileage with the diesel which was my primary reason for going that way. 19mpg on the only road trip so far and that was mostly 2 lane with a lot of passing on my part. Man, does this thing have a passing gear! Whoo Hoo! The best the 454 EVER did was 13mpg on the interstate. And the diesel will get better as it breaks in.
I've had no problems with my truck's drive line but I follow the forum listed above. There are/were some teething problems. The engine has a 5yr/100k warranty on it so I'm not too worried about it. The factory warranty, like most everyone else's, is 3yr/36k and very extensive.
There are a lot of urban legends (my brothers buddies dads 1st wifes 3rd husband had this problem... ) about the aluminum heads causing some problems or being problematic. As you'll see from the diesel forum, there are some guys on there that work for GM, Duramax and Allison as well as GM techs. As far as I've read, there are zero incidents of any head failures.
I drove the Ford F350 Lariat PSD back to back with the truck I bought. I liked the look of the Ford but the Duramax (300hp) flat spanked the PSD (250hp) in power. And the GM is MUCH quieter, too. The Dodge 4x4 full size trucks have a solid 'live' axle up front so I didn't even bother driving one. Good motor, very low tech and old fashioned driveline. The live axle was okay in my 87 suburban but not in an 01 truck. Independent front suspension, IMO and experience, is worlds better.
My advice would be to wait if you want the Isuzu. They have been having some problems that I'm sure they will fix. I own a 2000 F 250 4 dr 4x4 Powerstroke and have a Hot chip in it and some extras. I'm up to about 350 hp and the Isuzu was still quicker than my Ford. I prefer the solid axle for serious off road stuff and simple reliability, but the ride is not as plush as independent type. I love my Ford and I think it is the best lookin' truck out there. My advice would be wait, cuz competition works wonders for the consumer. Regards