I have tons of Quicktime videos on mine. Mp4 and m4v I use Real Player to download from youtube and it has a converter built in, but I use a Turbo HD device for big files. You can also use Handbrake to rip DVD's into playable format. I don't like the way iPad handles Photos or Movies, so I have iTunes Playlists, like, iPhoneMovies, and iPhonePhotos, that I drag the videos and picts into. Makes it easier for me to find them on both the iPhone and iPad.
Thanks very much!