GMT800's are the cockroaches of the truck world; they'll probably be here long after we're all gone - thanks mostly to the durability of the early LS engines.
I've got my GMT800 gripes mostly around electrical issues (they're way more complex than any GMT400) and probably more than half of them out there have an eternal CEL due to EVAP issues that are often more difficult to solve than they should be.
They're about as easy to steal as a GMT400 and are still high-theft vehicles to this day; the plastic exterior door handles don't inspire confidence. The thieves just knock the lock out of the handle; some of them just break the handle by forcibly yanking on it. Around Houston where auto theft is out of control, they even steal beat old work trucks. Get a Ravelco antitheft setup, it's about the only thing that will keep it yours.
Anyone living in rust belt areas will tell you the brake lines are made out of fairy dust that dissolves at the first hint of road salt.
They're good trucks.. styling is hit/miss depending on year and your personal tastes, to be sure.
Richard