I as well have been trying to figure out why the heck my 09 135i bounces all over the road. I'm coming from a full race suspension on a G35; that thing had a good amount of over-steer but overall it handled like it was on rails! When I'm semi hard in a turn with the 1'er and I hit a bump, even though the a$$ end feels like it's going to kick out, it always drops right back in place. The car is well balanced! My first attempt to fix this is a set of H&R springs which I just ordered yesterday. I also came across some tie bars on ebay. I know it's ebay but their function its what I'm concerned about, and for 130 bucks they're disposable. Having dealt with suspension and only suspension for the last 4 years I think the fix will be:
Semi thicker sway bar - Just to stiffen things up a bit, but now too much as the car doesn't have sway issues when not under load.
End Links - To accurately adjust pre-load on the thicker sway bars.
Springs - I agree with everyone on this... Linear over Progressive (unless you drive a 128, in which you should shoot yourself) ;-)
Rear tie bar, rear strut tower bar...
All of these should square all of the issues that most people are complaining about... I'll continue to post as I add these devices.