For me, the seat is the worst thing about the M3. Admittedly, I'm a fairly large guy - 115Kg, 1.8m and broad. I've covered over 20K miles in 6 months and I often have lower back pain that I have not had before from previous cars - Infiniti Q50, Audi A6, BMW 5.
I put it down to the length of the seat. It isn't adjustable to make it longer, so I don't get a lot of support along my upper leg. This seems to then cause lower back pain. I've fiddled with the seat and got it in the best position for me, but after a long drive, my back aches when I get out of the car. It isn't bad enough to make me dislike the M3, but, as with a few things in the car, it could be done better.