James...
I sold my 2001 2500 4X4 NV4500 4:10 5 speed last March and bought the 2005 in my signature.
I really like the 2001 but I needed more towing capacity for my 32' fifth wheel. I went with the dually for that reason. I ordered the six speed and was a little disappointed that the NV5600 was no longer available and that I would be getting the new G56 6 speed. I did a little research on this transmission and even talked to Peter (Southbend Clutch) and decided that worse case scenario he could fix any problem that might come up with the G56.
I now have over 9K on my truck and I haven't regretted one day selling my 2nd gen and going to the 3rd gen. Here are my +'s and -'s so far.
1. I don't miss the rattle of my 2nd gen and don't have to worry about lift pumps or injector pumps.
2. I didn't have to "chip" my 2005 as it has all the power I need for my towing.
3. The brakes are much improved over my 2nd gen (it only had disk brakes in the front with drums in the back).
4. The overall fit and finish on my 2005 is better then the 2001 and it is more comfortable to drive.
5. I like the G56 3:73 setup as it has plenty of low speed torque and hauls my 5th wheel just fine. I do wish it had a higher gear ratio in sixth gear as it turns out it runs about the same rpm in sixth as my 2001 did in fifth.
6. I would like better MPG (I've average just under 14MPG over 9K with only limited towing but lots of stop and go in the city). I knew the mileage would suffer with the extra two wheels and increased weight.
7. Even though it can't compare to the new MegaCab, the room in the back seat is far better then my 2001 QC was. I also like the doors all opening up the same way. I can't tell you the number of times I went to the store and had a problem trying to load groceries in the 2001 when someone was parked next to me.
8. I don't miss the messy oil breather tube plastic bottlevoir sitting just behind the fan and spraying oil all over the front of the engine. I eliminated most of this by extending the hose down lower and taking the bottle off, but I would still get a little dripping.
As I said before... . I don't regret the move up to the 2005 at all. It has more then met my expectations and the G56 has been no problem at all. We will see what the future has for this but Peter and Southbend Clutch already have a fix for those needing a stronger setup.
Analyze what you need, do some research and buy what will meet your needs and make you happy...
Attention: TDR Forum Junkies 
