I have been a Comcast high-speed Internet customer for about 5 years. The modem Comcast provided as part of a package deal was a Motorola Surfboard 4100. About 4 months ago, I noticed that I was getting barely more than 1/2 the speed I was paying for, which was 6Mbps. I complained to tech support. They sent a technician out and within 3 sec. after walking into my office, he said, "Oh...I know exactly what the problem is; you have an 'end-of-life' modem. You need to upgrade to a 2.1 DOCSIS modem, and I would recommend a Motorola Surfboard as one of the best." So I got on my favorite shopping site, Amazon, and bought one. Not only did it work perfectly from day one, my average download speed on Speakeasy.net is ~22Mbps -- almost 6 1/2 times what I was getting with the old Surfboard -- and the upload speed increased from ~364Kbps to ~1.6Mbps.
Would other brands do as well? Probably, but I stuck with Motorola because the last one was 100% reliable; never had a problem. The only reason it was not delivering the speed is because Comcast's system had changed and the old modem's software could not keep up with it. I also like the fact that Motorola moved the standby button to the top where it is more easily accessible. I have a new Vista computer and have to put the modem on Standby after putting the computer to sleep, because (not sure why) Internet activity tends to wake it back up.
I found it disappointing that Comcast apparently does not alert its customers when their modems are obsolete. If you're having a similar problem, check with your ISP. If you need to upgrade your modem, I highly recommend this one.