If you drive a lot at highway speeds it may be worth it, but for 55 and under, probably not. I believe a properly designed car will already have pretty good aerodynamics and the only separate "add on" part that is worth it economically is the cheap rubber front airdam because it has to be close to the ground to work and you don't want a permanent part of the body down there to possibly get torn off.
On the other hand, the more passion there is in the auto market for cool looking stuff, the more job security for people like moi. I ain't retired just yet.
Allan