I rarely need the MD but I have used it a few times and it worked fine.
When you use it, the drive train gets very spongy which pedals weird if you are used to the stiff drive train of the DF but I suppose you will get used to that and during climbing it's not so relevant.
I had a few technical issues with it
First time after maybe 2000km there was some play and clicking in the front. I also felt the clicking in my left feet. intercitybike fixed it by tightening a nut, I understood it was some bolt on the crank or MD being somewhat loose. I remember I saw a big special wrench they used to tighten it slightly.
Second time after about 7000, again clicking and scratching noises in the front. Intercitybike again fixed it, this time they said a nut on the MD that needed tightening to remove play.
Also according to slumpf it needs grease every 5000km, and only MOS2 grease (note, on one of their own websites it says you can also use other oils but not when you ask them in person!).
In all, the MD works fine but it is quite maintenance intensive compared to the other parts and it makes scary noises when it is not tight.