Well I hope you never do run into problems with the Curt or any other aftermarket hitch, because I'd bet anything that BMW would tell you your SOL if your frame gets bent/damaged and you better hope the next buyer of your X5 doesn't get it too thoroughly inspected prior to purchase.

I don't know who made the comment you referenced above (which was a little rude IMO) but personally I would never take that kind of risk with an $80,000 car just to save $300.