The app itself is a modest 113 blocks — in the UK, at least — but that makes sense in light of the fact that most of the legwork is done through online streaming; it follows the recent YouTube app update on Wii U.
Unlike on the Wii U the 3DS doesn't have a web browser capable of streaming the YouTube website, so this free app is certainly a welcome release. In our early dabbling we've been able to sign into our account, while the layout is generally similar to mobile apps and even the main site, except with menus and navigation condensed on the touch screen and videos playing on the top screen. You can see your subscribed channels, subscribe to others, like / dislike videos, add to your playlists and generally search and navigate easily; we can't find any way to comment on videos, however.
Our main observation so far, unfortunately, is that it delivers a low level of picture quality. Naturally the 3DS has low resolution screens compared to many smartphones and tablets, but even with that in mind the picture is heavily distorted and flawed, even with the "HQ" option selected. We've been unable to find any videos that play in 3D — as these support glasses-based tech rather than autostereoscopic 3D — and, in general, the experience is less enjoyable than even that on a modest smartphone. It seems like a last gasp option to watch some YouTube videos, rather than a go-to app.