I was allowing myself 1 bar of 85% Lindt per week at first (started paleo nov. 1st), but I noticed on two separate occasions that it sent me to the bathroom within 15 minutes if it was the first thing I ate. I seemed to be able to eat it with or after other food just fine, but by itself it seems to be bad news for me. Don't know if it's a gluten cross-reactivity thing or what, but it's been over 3 weeks now since I've had any and I was just starting to forget about chocolate altogether (until I saw your question!).
I like that there's a good bit of sat fat in dark chocolate, along with magnesium as others have mentioned. I did try the 90% which has a different but nice taste, too, though the one Green & Black's bar I tried was pretty awesome, but a bit pricier.
I'm considering re-introducing it after a few more weeks (with food) to see if I notice anything again. I could do without it entirely, I'm just curious more than anything.