I believe in the original Portal there was an achievement for knocking down all the cameras. It doesn't look like there's a related achievement in Portal 2. Does knocking down the cameras have any effect (aside from nostalgia?)
There doesn't appear to be an achievement for removing cameras in Portal 2. I wondered the same thing. It is, however, fun to watch them burn with the lasers. I don't think there was a fun way to destroy cameras and other objects in the original Portal.
It has no effect, and as far as I can tell GLaDOS never reacts to you breaking the cameras; there is a related achievement in Chapter 8 where you're challenged to break monitors, however.