It's been about a year and a half so far with Comcast, and I've had pretty decent luck. My first modem looked like it came out of the early 90s and was replaced after maybe a month or two. They could never figure out what caused my "micro-lag" (only lagging for 1-3 seconds at a time), which was rather annoying while playing online games. I eventually purchased my own Motorola Surfboard 6121 DOCSIS 3.0 modem, and the "micro lag" is gone.
Although, I had a huge problem last week where I literally dropped to dial-up speeds with my SB6121. I still had my old Comcast-provided Surfboard (5000-series DOCSIS 2.0) modem as I haven't had the time to return it yet, which I should probably do so soon... it's a $7 a month rental charge for that $50 modem. :\ But anyway... I hooked up my old modem, and the Internet worked fine. I put the SB6121 back in place, and the Internet was terribly slow. The following Monday, I put the SB6121 back again, and everything was fine. Comcast could never figure out what was wrong, and I couldn't either.
I believe it was Comcast performing DOCSIS 3.0-related maintenance though, because ever since I hooked the SB6121 back up, I get much higher PowerBoost speeds (5MB/s downloads). The only bad thing is that PowerBoost only lasts for like 50MB, so it's over pretty quickly.
EDIT:
As a note, even though I
do have DOCSIS 3.0 enabled for my receive (download), I don't actually get any faster overall speeds. With the SB5k and SB6121, I get 2.5MB/s sustained.