I've got a 1" spacer on my GT40/Explorer intake. It seems to keep the intake a little cooler when driving. But if I have to idle around a lot, everything under the hood gets very hot from the heat coming off of the headers. I installed the spacer mainly for the added torque it provides by lengthening the intake runners. I have my Mustang engine in a truck with an AOD, so I need all the torque I can get.
Most people just use it now for clearance for taller valve covers. I agree with Mike up to a degree on keeping the upper cooler, but after so many minutes of running with the same heat under hood, I really don't know how effective it is.