I'm using the factory BMW hitch without any aftermarket sway control.

FWIW - After looking at all of the available options and doing the hitch install myself and seeing how it is made an attached, my semi-professional opinion (as a mechanical design engineer) is that I wouldn't consider any hitch other than OEM for heavy loads.

Frayed - If you haven't wired in the electronic brake controller yet, you will want to check out my thread on the install. Wiring the the controller is super easy IF you know where to pick up the 12v braking signal.
http://www.xoutpost.com/x5-e53-forum/...hlight=prodigy