I just received a RV120W and my bandwidth rate dropped from 15-20Mbps on average to 1-2Mbps on overage (verified by internet speed test sites from my cable provider, cnet, speedtest.net). Directly connecting my laptop to my cable modem I get the 15-20Mbps, but through the RV120W the speed is 1-2Mbps. The cable modem is a Scientific Atlanta 2100, received from Time Warner Cable (new england) last fall.
Setup (no other routers, switches, etc):
Cable Modem (Sci Atlanta 2100) -> RV120W -> Laptop Connected to LAN1.
Note that I do not have any logging or other diagnostics enabled and that I have not changed any of the configuration out of the box.