Forgive the thread bump, but did anyone notice one of the recent versions of iMovie added an adjustment slider for video effects? Now you can control the intensity of a video filter or effect, compared to past versions (since iMovie '09 first added such video effects).
Of course you don't get various effect perimeters like in iMovie 3 to HD 6, or keyframe control like in Final Cut Pro, but this is definitely a good thing for beginners or anyone making fairly simple video projects, giving them a little more control over yet another aspect of their movie.
Excellent tip, very perceptive of you!