Just like enkeia says it depend on the area you are in. I find it works best when your parked or traveling in an open area, and not so well on tree lined highways or in the city. You have to remember it's like driving a big rotor-antenna your reception is going to change as you change direction.
I have a Pioneer P6400CD with a TV tuner, the installer chose not the use the give me antenna and installed a Accele TVA40AS single amplified antenna, can't tell you if this is any better since I have nothing to compare it to.
Over all I am happy with the reception, it is what it is.