I used this GUI quite succesfully since it first came out. I also used the Mac since 1985 and although I am no programmer I never noticed any particular problem in my machine nor any other side effect. I deal with sound, graphics and DVD files, and I also keep my mail and all in my machine. No problems so far from FFMPEG. I don't know if it is an Apple certified piece of software, but it does what it promises since the early days, and I never had problems on the side.
Perhaps this GUI allows an experienced user to do far too many things, and a proper manual would be very welcome. However test and trial got me all I need so far, perh0pas I had to try hard at times but it worked at the end. If I needed help I got help from the programmer quickly.
So I am really pleased with ffmpeg: all my valuable data relax in my machine mainly because, as a filmaker and editor dealing constantly with VOB, MOV And sound files this GUI helps me to my work done efficently.
Other far more expensive software does not work, and most of all costumer care is null in many cases.
It annoys me to read simplistic comments especially when they come from people who seems to know how to deal with OSX, while I know nothing about UNIX, and anything wich helps me to use its potential is a blessing to me, especially when it does what it promises.