On official iOS:
For videos...try tagdisk hd+ in the app store...there is a free & paid version.
For audio copy the youtube link/URL into:
http://www.youtube-mp3.org/
paste the link into the designated box.
Click on ‘Convert Video’.
Afterwards, the site would process the video, then convert it to MP3. All you have to do is download the file and you are done!
You may have to use a 3rd party browser with download capability....iCab or atomic browser are best for this.
On JB iOS:
For videos:~YouTube HD will integrate into the native YouTube app with a downloads tab on bottom.
For audio ripping:~ use YoutubeToMp3 - copy link/URL into field and let it do it's thing..then you can import into iPod app.