I install this modem easily and called Comcast, who took 15 minutes to provision the modem.  Directly connected to my old Belkin F5D5530-W modem, I got about 6.8 Mbps download and 1.3 Mbps upload, according to [...].  After installing this Motorola SB6120 modem and connecting directly to it, I got 21 Mbps download and 4.8 Mbps upload.  What a big difference!  I only have the "Performance" high speed internet service, which is supposed to be 12 Mbps/ 2 Mbps (up to 15 Mbps/3 Mbps with PowerBoost).The downside is that when I connected it to my existing Belkin F5D7230-4 Wireless G Router, the speed decreased to 13.9 Mbps / 2.8 Mbps.  Does anyone know if the router is causing the speed to slow?  Would a newer router help?  Any tips on what to do?Although I'm not using the modem to it's potential at the moment, the speeds are still faster than they were before.  The modem is worth the money I paid. 

Question: How is the speed?
Answer: I got about 6.8 Mbps download and 1.3 Mbps upload