In terms of powertrain and reliability, it's just as good as any other GM pickup truck. In other words, best in the industry.
However, the question is, why buy that vs a Silverado?
It tows fine and and handles well for a truck. It's more or less a Silverado with interesting styling and less engine choices.
Modern ones (2007+up) are great but I know the older models had issues with the plastic cladding, but the dealer offers solutions to those problems (mostly haziness on the plastics).
It has that whole Truck/SUV in one package deal/gimmick. In my experience, none of my friends or relatives who own an Avalanche really use that feature.
I would buy one for the styling because I'm a fan of it.