Wife has a 2004 Z66 Avalanche, she absolutely loves it.
Usual problems with the gauges and ABS stuff. I've had the gauges done so far, but the ABS computer will wait until my new carport is completed. The gauges ran $150, I had it rebuild and swapped to blue LEDS. The ABS runs $100 for a rebuild and $300 for new, ours is the NW7 traction control version.
Another point is the seat covers, which are moderately easy once you pull the seat out.
I built a pull out drawer for her, best upgrade for the Avalanches ever! Every time we use it, some people at the store point and whisper back and forth. You know the wife is telling the hubby she wants a drawer for her truck. Built it from plywood and some appliance rollers, then painted with deck paint. You can see several examples on the different Avalanche forums.