I'm not a huge fan of the Youtube update. The cards look flat and feel static. They don't have any of the actions you might expect from a card such as swiping away or expanding to reveal more options, nor do they have any cool animations like the cards in Googles Now and +. Compared to the latter they also have more padding around them for some reason, lending to their bloated appearance. I think the previous feed views were much more elegant with the text and user icon overlaying the video thumbnail.
I also found myself foiled by weird app navigation twice in my short play-around. Firstly you can't open the navigation drawer while on a video screen, not even with an edge-swipe. Second, if you click on the uploader's channel while watching a video (thus minimising it) then press the back button, instead of maximising the video again it will take you back to whichever non-video screen you last viewed.
I am impressed by the animations when swiping the active video around. Not sure it's a feature I'll use much but it's hard to know without trying it out for a while.