Worst game of the decade? Hell no. Worse than it should have been given both the people behind and the amount of time it took to make? Yes.
For me, its good moments were heavily overshadowed by the schizophrenic mish-mash of level design, boss battles and combat design. It was as if they took a great premise, but messed it up by giving it a bad formulaic and boring story, and then made it worse by putting almost every single bad idea the FPS genre has ever come up with since 1997.
Even the 'move this thingy over to that thingy by pushing it because you can't pick things up' mechanic from Prey (which I hated that mechanic in Prey, and I hated it in DNF) made it in there.
Overall, the game is okay at best, but the bad design choices and mish-mash of different design choices coupled with the 12 year wait make it a bad game.
People talk about how it's unfair to judge the game by how long it took to make. Sorry, but that's BS. If a game takes 12 years to make it better damn well better be worth those 12 years. Sorry, but that's just the long and short of it.
If you liked the game, that's great. But I certainly wish I was able to get back the few hours I spent on it. I wasn't even able to finish the game I hated it so much.