Is it worth putting a spoiler in my car?

i have a 96 jetta gl...its a nice car,stick shift and i was wondering if it was okay to put a spoiler in it since i understand its a 4 cylinder but im always on the freeway n stuff at high speeds.the car has like amazing gas mileage and i hear if i install a spoiler it will have the gas milage of a hummer...which is awful.so is it okayy?will the gas mileage become low?pros and cons please?