I fitted one to my e36 M3 half way through a track day years ago. Turn in was noticably better and lap times reduced by 0.5secs. Not sure you would really notice on the road and the newer chassis are much stiffer anyway.
I do have one of these on my e82 125i though.
2014 M235i / EB / Coral / Sunroof / Comfort & Parking Packages
2014 X3 3.0d / M sport / AW / Sunroof
2009 125i Coupe / M Sport / AW / 6MT/ Sunroof / 261 / BMW Performance CF Spoiler, Stripes, Strut brace