I have the front and rear M3 bars (not sure E90/E92/etc). The rear is too stiff on the track which makes it tricky for a less experienced driver; the car will oversteer before it understeers.
We are RWD; just use the gas pedal to bring the rear around, you don't need a stiffer sway bar to reduce grip back there. Only on FWD cars does it make sense to do a stiff rear sway, they don't have many other options to help bring the rear around.
Edit: also it hasn't been mentioned, but installing the rear sway is expensive because it requires lowering the subframe + mandatory alignment. I would replace my M3 bar with the stock one if it didn't cost so much.
2011 BMW E92 335i - Le Mans Blue/Black, 6MT, M Sport package
Last edited by CJ421; 10-08-2012 at 11:23 AM.