I have problems with the DVR functionality after upgrading to the Quantum box.
For example, in the old system it was easy to watch a recorded show and switch to check on Live TV for a score update and then switch back and continue with the recorded show. I just used the "last" button. Now, the last button brings you to the live TV, but the next push sends you to the second to last Live TV station, not back to the DVR. Now if you go to Live TV and want to get back, you need to hit: DVR, OK(to select recordings) and the scroll through the list to your show, then select OK, then scroll to select "resume play" then OK. Similarly, if you are just cleaning up the recorded list and delete a show that was not in a folder, you are thrown back to the main DVR menu. You then have to go all the way back through the list to continue cleaning up the recordings. In the old system, after deleting the show, the focus of the system remained in the recorded list (below or above the entry you deleted, I don't recall which).
Also, with the FIOS Voice functionality, after playing a voicemail you are returned to live TV, even if you were watching a recorded show.
There are other issues as well.
Extremely poor End User Experience.