From the Store description:
myTube is a YouTube app built to keep your video playing no matter what. By lightly fading the video into the background, you can continue to watch while you read comments, browse channels and videos, and everything else you can do in the app. If you can do something in the app, then you can do it while watching a video! Want to go do something else? myTube can play the audio of videos in the background while you use another app ;)
- Watch video while reading comments and browsing, and even listen to it while using other apps
- HD video support - Rate, comment and share videos
- Browse playlists, and add videos to your own
- Browse and subscribe to your favorite channels
- Save videos for watching when you have a limited connection
- Pin channels' uploads, favorites and playlists to your start screen
- Double wide live tile
- Offline playlists
Here's the changelog for the new v2.x since my previous story, a year ago:
- All new design
- Ability to play videos in 1080p, 480p and 60 FPS
- Ability to download only the audio portion of a video to save space
- Ability to save videos to the SD card if your Storage Sense settings allow it
- Open videos from the browser in myTube using IE's share option (no need for the "Open in myTube" link, also works on non-YouTube pages that have YouTube videos embedded)
- Support for signing into multiple YouTube accounts
- Ability to view liked videos
- Watch history now loads much faster
- Videos start playing much faster
- Ability to browse the app in landscape
- Background downloads no longer have the file size or connection type limitations that were present in Windows Phone 8.0
A few screens of the new version in action on my Lumia 830:
Note the cleaner design and note the smaller 'music only' playback icon - this will carry on in the background even if you switch to another application...
The main USP of myTube! remains - having the UI float on top of a playing video, so you can browse comments and anything else you like while (nominally) watching the video play; (right) there are save options - for archiving footage for watching later, offline...
Ah, seems this video was only uploaded in 720p, but there are 1080p download options too in myTube! (right) the save 'manage' view, also superimposed, gives an idea of how your downloads are coming along....
Although I presonally prefer MetroTube, there's lots to recommend myTube! too (I bought it). In the meantime, you can try (there are some limits in how much you can watch per three hour period etc.) or buy myTube! for a nominal cost ($1/79p) here in the Store.