I gone with this mod in the past, and I wasn't overly impressed with the performance of it. The sound in the Xterra isn't exactly sterling to begin with, but the FM Modulator way really kills the sound of satellite radio.

I am currently a Sirius subscriber after having had XM for a while. The quality is the same on both, and dropouts are not that big of an issue with either of them. But, I did get away from the FM modulator and go with a new, dedicated head unit.