I am of the opinion that an EQ is instrumental in car audio. The car is far from being an ideal listening environment, and you have to compensate for alot inside the car.
You have to contend with outside noise,resonant frequencies within the car, imperfect speaker enclosures, speaker placement, etc... You'd be amazed at what good door and cabin preparation and equalization will do for sound quality.
Anyone on this board that has heard my system will vouch for that. (I'm not trying to brag, just trying to make a point. :p )
I am using a Phoenix Gold 215ix Equalizer, and I love it. It really has made all the difference between having a mediocre soundsystem and a great soundsystem.
If you are going to spend money on an EQ, I would suggest getting one that is versatile and precise, preferably trunk mounted,with multiple bands, and separate controls for both left and right channels.
Once it is set, you shouldn't have to go back to it. Like Dnewma04 said, set it and forget it. BTW, I find that when you resort to the bass and treble settings on the HU, it "muddies", or colors the sound. The adjustments are much cleaner on a good quality EQ.
My 0.02$.....
