after driving more with the sport button on and off, I can tell the difference now... I can't exactly feel it , but I can hear it in the bumps in the road. With it off, it is definitely easier on bumps, a bit more give (though it does sound unstable and all-over-the-place under there). Don't know exactly how to describe it. I don't race the thing so couldn't tell you anything about how it corners while doing 70 with vs without sport.

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr