I live in Seattle and have taken my '06 X5 4.8iS with 20" Diamaris up to Snoqualmie Pass twice so far and it's done great. Stopping and turning are the weak points, like everyone's said, but working the transmission seems to do pretty well for me (2nd to reduce speed, then neutral to stop).

That being said, I ordered a pair of Spikes-Spider Sport chains and just installed them on the rears today. They are the ONLY chains that you can put on this car, and at ~$465 a pair, they aren't cheap. But the quick install and good build quality help make up for the price, and it's a cheaper and more convenient solution than having a separate winter setup. I'm switching to all-seasons when these tires wear out though.