Compared to our current ML, I personally love it. It's definitely way better than the 164 quality wise. The plastic feels and looks different than the W164's. I hate our cheap, plastic-y dash (love the leather dash in the ML63), the standard dash in the 166 is way better and I love that slap of wood, which the 164 lacks even with a full leather dash.
Hated the location of the seat controls and turn signal stalk, now they're where they belong on every Benz. Controls and switches on the inside seem to come from the E & C or they seem to look and feel like the ones in the E & C, unlike the crappy ones that are currently in the W164.
The door handles have those indented touch sensor "lock buttons" like the S instead of those cheap black rubber buttons that would come off after a while. Wish the doors had that MB tank thud when closing the doors. Love the large sunroof (like our X5), but this one didn't have it. Love the front (and back) bumper with that chrome skid plate, and the standard grille isn't as cheap looking as the one we had on our 164 (which I couldn't stand and promptly replaced).
Again, rear legroom hasn't improved. I'm 5'-9" (175cm) and to me it's still the same uncomfortable place to be just like our W164, the X5 still definitely has better legroom back there. After noticing the huge difference between the 163 and 164 in ride quality, there's definitely a huge difference between 164 and 166 quality wise.
You will love your 166, I can't wait to replace ours.