Vince, the fan works OK and I really notice the difference - especially on hot sunny days. Some people may actaully not like them becuase depending on the model they may be a little bit of a hassle to mount on the window. Also, some models have pretty weak photovoltaic cells and the fan will not spin unless its hit by direct sun light.

The key is to avoid the cheaper units, and it should work OK even on cloudy days. Mine will not allow bugs or rain in when mounted properly.

MSantos