I just came back from a weekend driving through the Rockies in the X5 and I'm very impressed. This was the first trip of any length since I bought it last Sept. The X5 was stable, smooth and confident even at speeds of 180kph. Passing was a dream, especially on uphill climbs. It handled the long sweeping turns in the valleys and the tight S-turns in the passes with equal ease.
One complaint is that I wish the 4.4 came in a manual. It seems that the auto, even in sport mode, gets in too high a gear at times for the situation. When the road has rolling hills that go up and down the tranny wants to shift up to 6th going downhill and is in the wrong gear to climb up the next hill. I end up having to kick it down a gear with the pedal or put it in manual mode. I hate driving in manual mode - I have to fight the urge to clutch on each shift.

My 330xi 5sp is my daily driver so I don't think I'll ever get use to auto-manual mode.
I also wish the X5 had more cargo space. With 2 young kids we end up taking a lot of stuff on road trips. I bought the BMW roof carrier bars but I haven't picked up a cargo box yet. Any recommendations for cargo/ski boxes? Thule or Yakima? The BMW boxes are nice but really pricey.