I think it depends what you want to carry. Brand name racks like Thule and Yakima will easily handle four bikes, a bunch of snowboards or one of those big storage boxes. Lesser racks may not be able to take the weight or withstand the lift (yes, lift) that some items can cause. It could damage your car if it's not up to the job. I've seen it happen -- I used to sell these things.

That said, even a cheap rack should handle one or two bikes, or a couple of pairs of skis.

If it were me, I'd fork out for Thule rather than risk my BMW.