Go for it! The first thing I did when we bought our van was to tint the windows, now all the rear windows are a dark factory tint, and I used a light tint on the front, so you really can't tell.
I plan to have my rear window done since it will be out of the car for paint, still undecided about the sides, but most likely will do them since it will be a summer only car. There is a place local here that will either spray them or use flim, which is better?