I went and picked up a DCX3400 today. Actually, I had to do it twice because the first box wouldn't authenticate, but the second one works and I am very happy with the new box's performance and increased storage - 250GB hard drive, up from 160GB in my previous DCH6416.
Switching channels is noticeably faster, with no long pauses or lag between the button presses and the box actually switching channels. Once in a while I do get a brief flash of garbage on the screen before the new picture kicks in, but it's a minor problem to me and I can live with it. Someone mentioned a problem with audio occasionally not coming in right away, and I've noticed that on a couple of channels. I'll change channels and while the video comes on right away, it takes several seconds before the audio comes on. It's only happened a couple of times so far, and I'm waiting for the programming guide to fully load up before I play around with it some more. I'm especially eager to see if the DVR programming fiasco is somehow dealt with and fixed on this new box. I hated the daily routine of deleting scheduled recordings of reruns which shouldn't have been queued in the first place.
Anyway, if anyone is wondering whether the switch is worth it or not, I'd say go ahead and give it a shot. The box is smaller, faster, and has more storage than the previous version.
One note: Although the new remote looks the same as the older one, the customer service lady at the Comcast office made a point to make sure I got the newer remote (it has dark grey finish, as opposed to silver).
Newer model is on the left. And I did wear out the FAV button on my old remote because I programmed it to do a 30 second skip.
Apparently, some of the key mapping are different. I don't have specifics, but I do know that some of the offline command keys work on the new remote but don't on the older model. So if you get the DCX3400, make sure you get the new remote with it.