Yes, I've done it.
Dark difficulty. Three words: Not that bad.
Seriously, Dark was a breeze for me once I learned that Quen is essentially Godmode and it's not always the best choice to go in head first screaming THIS IS PONTAR VALLEY.
The number of missables are insane and you actually need to know of some of them before hand otherwise you WILL miss them. On my first playthrough I missed 4 achievements on Roche's path and didn't do enough quests to get to level 35. I finished my second on Dark and then proceeded to get everything I missed in about one hour. It's surprising how quickly the game goes if you skip all the conversations and side-quests.
Anyone else have it or are going for it?