It looks like things are a little better for DKs now, but still not ideal:
I have mained a DK since WotLK. I started on my Monk for Legion primarily because I played my DK in the Beta and didn't want to start Legion doing the exact same things on the same character. But now just a month past launch, I still haven't leveled my DK past 104. One factor is that I've been working on achieving Pathfinder (which I just did yesterday on my monk) but another is the lackluster performance and lack of raid and dungeon utility (only Blood kept Gorefiend's Grasp) is also impacting my motivation.
On one hand, I understand that balancing this time around is much harder, because of artifact skill trees, completion %, and legendaries. But it also felt like DK was underachieving for most of WoD, so I agree with JFrazer that's it not the most auspicious sign.