i do not have this installed on my crossfire, but have had experience with FM modulators. pretty much it depends on how much you care about sound quality. although there are some good quality sounding FM modulators, they still are only as good quality as a clear radio station. if this is fine for you then an FM modulator is the way to go.
i had an FM modulator installed in my old acr (cavalier z24) in order to input my play station sound into the car speakers (when i didn't have an aux input on my headunit). i bought the FM modulator from best buy (it was ok quality as some can sound really bad) for around $30. for my needs, it worked fine for playing video games and not so bad for playing videos, but i care a bit about sound quality for my music. so once again, if you are ok with the quality for a really clear radio station then the FM modulator would work for you.