I set up two Nanostation M5s this spring. They are about 4 miles apart. Got one on the shed roof on a 15 foot mast, the other on another shed on a 20 foot mast. I can make them talk to each other in the winter when the leaves are off the trees. But once the leaves start to grow, the conection is gone.
 One thing to keep in mind with long distance transmission is ther Fresnel zone. Eventhough my antennas are high up in the air and one would think they would clear the trees, you loose a good part of the signal from the lower fresnel zone.

I have attached a pdf that was sent to me by Bill Moffit of Ayrstone. It explains how the Fresnel zone affects transmission. I hope Bill doesn't mind. 

range and line of sight.pdf (176KB - 471 downloads)