Just test driven a 550i GT and X5 35i back to back.
I understand that GT generates some very polarized response - either you love it or hate it. But it drives pretty well. Though you are aware of its weight, it handles beautifully.

One thing I really love about the GT is the black panel dash and the interior. The dash is completely black apart from 4 partial rings when off. When you open the door, the numbers light up and the rings are completed. The lower part is a high res color TFT display. Really looks great. Wish they had included updating the orange-dot-matrix display in the X5 when doing the facelift.