I used it a bit, and I really think the developers dropped the ball on this one. The UI looks nice, but no amount of aesthetics can replace how awkward and difficult it is to use. You have to actively use both the GamePad and the TV screen in order to search for videos, and, as far as I could tell, videos could only be played on the TV, not on the GamePad. Granted, I only used it for a couple minutes and only watched 2 videos, but the reason I didn't do anymore was a combination of sleepiness and just a total dissatisfaction with it.
I know YouTube isn't a necessity, but if you're going to have it as a native app, have it right.