I have owned all M3 but an E30M and have an e92 now. I love the motor but I am looking forward to the new turboI6. Unlike most here I am not too concerned with it not being a completely new motor. I have no complaints with the 1M or 335i motor though I have never owned a car with either the N54 or N55. The mpg will be a small bonus but with the way I drive the car I doubt I will average much more than the low 17 I do now. Power delivery will be great and the torque will be a new experience for me. And if they break 7500rpm I have no complaints. Overall I am sure the motor will be fine and I will like it.
What is drawing me towards the new M4 is the technology. I have always driven manual so it will likely be my first DCT too. I look forward to the weight loss the most. I am hopeful for a full 150-100lbs though I doubt we will get that. Also I suppose that new trick M differential from the f10/12M will get carried over into this and it seems like a pretty cool thing to have. Also finally having proper brakes is a nice touch.
I want the new M4 because of the sum of it's parts and what it is. I could just buy a 997 GT3 if I wanted to go fast with a manual and high revving motor. And admittingly I will miss this. But I would like to try something new. Besides I will still have my E92M anyawy lol (unless R8 V10 depreciate enough in the next 3 years lol)