I have iMovie for iPhone and like it a lot. There are a number of tips in my sigline for using it.
When I had my iPad I bought ReelDirector and used it once. There are more options with it but it was just UNBELIEVABLE that you cannot run your edits without rendering (exporting) the file first.
and it takes quite some time to render a full video. The sample video it came with (a tutorial) had to first be rendered to watch it. Took just over 20mins.
so guess what? don't like it? make your changes and re-render!
more than enough for me to shelve it. I make a change, run that change a few seconds after and before, then move on to the next change. Impossible to do with ReelDirector.