Not having experienced the DL lie-flat BE seat but having flown and slept many nights in the A330 and 747 WBC seats (lie flat as well, just slightly angled - but so is actually good for health and coincidentally happens how I was recommended to sleep by two doctors), I would say that really instead of putting in the flat un-angled BE seats, DL should reconsider instead putting in the NW and KL style WBC seats. They are well proven to work well and be comfortable; no wonder that both NW and KL selected them. Sure, the slight angle can be annoying if one has slippery clothes, but other than that it's great for both sleeping and relaxing. The pod gives good privacy - just enough, but it's not as much a "coffin" feeling like it seems to be in the DL BE lie-flat un-angled seats.

Personally I always try to avoid DL aircraft (many 767's on long routes are really, really bad cabin interior and lack of IFE wise and seemingly have been in need of a cabin overhaul for over a decade, and the lie-flat un-angled BE seats have never seemed very appealing to me and seem to close to each other - one is sure to hear the person next to one snoring, it seems) and try to fly on the NW 330s and 747's instead whenever possible.