It depends on the install and what music you listen to. If you get one, make sure there isn't a crossover before it, or it will sound like crap. It works well for classic rock and new rock, but it made bass music or rap sound terrible. I had better results with it when it was used in a sealed box, but even then it sounded a little fake.
I'm sorry I didin't read the question right. The info in my first post was about the epicenter. All of the "bass enhancers" work kinda in the same way, except for the epicenter. It has many patents on its design that the others can't duplicate. And the epicenter comes with a five year warranty and just feels solid. I would say they are cheap knock offs.