As a stand alone series .Hack//DUSK may have been alright. However it isn't stand alone, it bears the .Hack name and did a poor job of fitting in with the family so to speak. Based off of the Manga it should have done alright. It had a good story laid out for it to follow, however not long after the frst couple of episodes it decided to stray from that path and follow a different direction, which, in my opinion, was a horrible mistake.
While some cool characters were introduced as a result of this divergence it as a whole ruined the story, and made it so that the watcher started to wonder if what was going on actually mattered. And don't even get me started on the bull shit ending the writers pulled out of their ass.
The animation is beautiful and the .Hack universe is very compelling, but in summary, there's a reason why this series isn't considered canon anymore.