I totally agree with Jack and Rolyasm.
The buttkickers can greatly increase your movie experience. It does not matter how perfect your subwoofers are, and it does not matter how low the subwoofers can go. You cannot really hear anything below 16hz anyway. The buttkickers are great complement to the existing subwoofers in your system. The key here is a properly tuned system. You do not want to have the buttkickers punching and shaking your chair destructing you from the movie when it is not necessary. The properly tuned system will only activate the buttkickers when intended. If your system tuned properly when the bullet hitting the wall, you may feel that it just went through your body. When there is an explosion in the movie, you can have a feeling that your house is coming down.
I have 8.4 system. Yes, 4 subwoofers, and they are pretty good ones
. I still love the buttkickers. If I turn them down, I feel that something is missing from the movie...