I have the OEM carpet mats covered by the OEM rubber mats. I actually don't care for the rubber mats. I have had these:
http://www.autoanything.com/floor-mats/79A4424A0A0.aspx
in two other cars and should have gone with them again in the BMW. The Lloyds are thinner and more flexible. They also trap moisture and debris from your feet really well. For $50 a set I highly recommend them.