BMW sells a rebranded version of an aero bar, for more than an aftermarket one will cost. Buy it if the label is important.

I like the Thule 450 Crossroad, and have used them on 4 vehicles now, includes an X3 and an X5. They use rubber attachment feet and so don't mar the rails, and they are rectangular, and so are very functional. I don't find them noisy in the wind compared to what I mount on them (with two bikes and two wheels I do hear some wind noise). I like the rectangular load bars because any attachment mounts to them easily. I don't leave the rack on. If you do plan to do so, you may want an aero bar for lower wind noise and looks. They just give up some functionality because you have to make sure any attachments (bike holders, etc) work with the aero bar you select.

Most people never use a roof rack, but at least the X5 comes with rails that are functional. I don't like racks that have to clamp on to the roof or gutter.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue