Sibelius 4 now comes with the ability for you to play a video file along with your score. so you can time the music to the video... handy for film composers as well. and say, incidental music? (is that what its called?) like the music you hear when tom runs after jerry.
I think this software is probably target more for film and media composers wishing to build mock ups. For notation I believe Sibelius is the way to go. If you need a cheap orchestral library go for Garritan GPO.
how do you like garritan?