I received my 2650 on Saturday and had it up and running in about an hour... I have all of my stereo equipment in a built-in cabinet next to my TV and I was out of outlets and cable jacks so I had to run a splitter and install a second power strip.
After hooking everything up, I went through the Media Center setup guide and I think it failed to recognize the tuner the first time, but eventually it got it. I called Comcast to activate the cablecard and the lady on the other end sounded less-than confident in her ability to do so. She did ask me for all the numbers that were on the screen so I was optimistic that this would work. After I hung up, I finished the set up and immediately got the guide and some HD channels.
Unfortunately, I wasn't getting all of my channels... I was getting ABC, NBC, CBS, etc., but not A&E, MTV, TLC, etc. Also, the picture looks really stretched out on the screen. I wasn't sure if it was a non-HD picture, or if the picture was so stretched that it appeared to be non-HD.
The next day I called Comcast and got transferred to the cablecard department. The guy wanted to confirm that I was using a Tivo and told him I wasn't, I was using a PC. I sat on the phone with him for a while while he went through some menu and answered a bunch of questions. He only asked me two or three questions though, one of which was what operating system I was running. After sitting on the phone in silence for 15 minutes he told me that he was at the end of his "walkthrough" and that he couldn't fix the problem. I made an appointment for a Comcast guy to come out, because that was the only option he gave me. After I hung up, all of my channels magically appeared, so I'm assuming something he did on his end resolved the problem.
The next problem related to the aspect ratio had to do with the settings within WMC. I was attempting to change the aspect ratio on my TV and the computer's settings without any luck. I also went through the set up menu in WMC several times, but nothing seemed to work. Eventually, I noticed that if you right-click while the live TV is playing, there are zoom options and for some reason, some of my channels were at "Zoom 2" instead of "Zoom 1." I changed one of the channels back to Zoom 1 and that seemed to fix all of the other channels as well.
I'm still working out some kinks with regard to the DVR features, but so far (knock on wood), it's worked pretty well. I love the WMC interface. It is so much sleeker than the ugly Comcast menus that look like they're from 1998. Also, the onboard DVR function is wonderful. I never had a DVR from the cable company because it was so expensive every month... gotta love free!