You have multiple options:
MXTube - FREE
Offline storage in var/mobile/media/mxtube, so you can extract saved movies later from your device. Upside, it's low key and small foot print make it a good quick "go to" program. Downside is it's a seperate program from the native YouTube.app that comes with the iPhone.
YouTube HD - it's NOT free, can't recall price at the moment
Same as MXTube, except this integrates into the native YouTube.app. I personally have had issues with this, most notably, after saving a movie and rebooting the iPad, it disappears from the list of downloaded/saved movies, however when I SSH into my iPad, the movies are there. Bug in the program or just my iPad, I don't know. 4.3.3 / JBMe3 - using latest version available as of posting date.