I have been using SimpleMovie for a few years now, and I could not be happier with its progress and development. It has become faster, more stable, and more feature-rich.
I use SimpleMovie to do all my censoring of R-rated movies. I've become so good and fast at it, that I have friends and family grabbing my edited movies on a daily basis. If it were legal, I would start a company, and 'simply' use SimpleMovieX.
The power of SimpleMovieX is best demonstrated when I edited The Departed. I was editing a 2 GB mp4 file (.avi is always a disaster waiting to happen), and I quickly took out 256 f-words, and a hundred or so more inappropriate terms.
It took many hours, but SimpleMovieX was reliable, stable, and easy to use.
Movie files are sometimes quite unstable, especially synchronization issues. However, I have been impressed with the sync tool within SimpleMovie. It isn't perfect, but it's the best solution out there without converting the movie file a 3rd time.
Also, Benoît Joossen (the creator) has been fantastic. One of the updates created a new style where the selection didn't follow the playhead. I sent him an email, and within the day, he had a new version of SimpleMovie with a preference in the Editing tab, where you change it back to the old method. Talk about customer service!
This is one app that is very worthy of its price. You will get great support with constant application updates and enhancement.