I installed one after I lost a Koi to a heron.
http://www.kissingfrogs.tv/scarecrow.html
Haven't lost a fish since. That's the pro.

For cons, if it looks into the sun, that can cause it to go off
falsely. And, I have seen a heron land out of range and then walk all
around the pond without setting it off. It moved so slowly that its
motion was not detected. Still, I haven't lost a fish since
installed it.

Steve J. Noll | Ventura California (USDA Zone 10)
| The Glass Block Koi Pond/Fountain:
| http://www.kissingfrogs.tv