I've never been a fan of fake owls. They rarely do the intended purpose of "scaring" birds away. Birds either figure out that they are fake or it can attract crows...or the occasionally very dense Cooper's hawk: [youtube]http://www.youtube.com/watch?v=k0aRlPCLp70[/youtube]
We sometimes use fake owls to get the attention of passing raptors to come down to the nets during hawk trapping season but they generally figure out their fake in less than a minute. According to the description of the video, the Cooper's hawks were nesting and kept trying to drive off the owl statue from the nest...but you would think that they would figure out the bird was fake. It also says in the description that the owl was removed because the hawks simply wouldn't leave it alone.
I love the accipiters, but they don't always think so clearly.