I don't have specific experience with a lightened flywheel with a BMW, but when I put one in my VW it completely changed the character of the car, and for the better. It revved much more quickly and it actually increased drivability (for me). Double-clutch downshifts (5th->3rd) were just a tiny blip of the throttle away. It seemed to make the car faster off the line as well, but that could have been my imagination. I would do it again on any 2.0L vw that I would own.
The problems that I would have with doing it on BMW's 2.5L I6 center around two issues.
1.) BMW provides a dual-mass flywheel, presumably to reduce harmonic engine vibrations. If you remove a dual-mass flywheel, you MAY have unacceptable engine vibration.
2.) I experienced a problem just recently that may perhaps be due to overheating my pressure plate and/or flywheel. A lightweight flywheel would overheat more quickly than a normal one. However, the flywheel may have not, in fact, overheated. So, this could be a moot point.
These are the issues that I would discuss with the aftermarket vendor of the lightweight flywheel. Guys that provide these items should know enough about them to honestly discuss the pros and cons. For $400, if they want your money, they should be able to talk about it. Personally, due to my good experience with one on my vw, I would be more likely than not to give it a try - if they can talk about my two concerns.