I've heard numerous times from numerous people saying the 20" summer tires were basically sleds in the snow. As in your life would be in danger type of useless. Do what a lot of people have already done and don't order the 20s from your dealer. Do it aftermarket where you can get all-season 20s on and drive with them full-time. Swapping twice a year = pain in the friggin rear.

In contrast, I've heard from more than a handful of 20" all-season owners that they are perfectly fine in the snow. I was debating this myself for quite a while. You want Michelin Latitude Tour HPs or Toyo Proxes S/Ts. One other benefit (in my opinion) is that these non runflats give a MUCH softer ride (coming from runflat 19s).

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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