DeepOvertone - see my post
http://www.1addicts.com/forums/showthread.php?t=786982.
The M3 front suspension adds some very nice steering feel and response - but the bushings are HARD. You really feel a jolt when going over any kind of sharp edge (uneven concrete slabs or metal strips). Not sure I'd do it again.
The M3 rear subframe bushings are virtually mandatory. They're expensive (generally 6 or 7 hours of labor) but make a huge difference.
Just picked my 2013 128. When the suspension gets too soft (as it will) at 30,000+ miles, I'll do the rear subframe bushings again and put in a set of Bilstein HD or Koni Yellow shocks.