I have one and it works pretty well. There is a guy (William Cowan I think) that long ago posted to this site (the old one) that has web site and on that site he details some mods he made to the unit. I have made those as well. The big one was a gain issue. The unit would not generate harmonics until the input level reached a high enough level.
I analyzed the unit a little using True RTA. What I found was if you ran a signal of say 30 Hz into the unit, you got your original 30Hz and 15 Hz. Both levels are controllable. The other thing you got was higher harmonics like 45 Hz, 75 Hz, etc. This I believe is due to the use of a flip-flop (digital device) to divide the frequency. The problem is the output of the flip-flop is a square wave, and square waves have odd harmonic multiples.
If you use this unit, you should run it before your BFD, and you might want to add a filter to the BFD to act as a low pass filter (xover).
I would have to go back and see how I hooked it up, I did not use the sub output; because, if you bypassed the unit, your base was gone. I think I used the satellite output full range.
This unit does nothing at all with content below about 20 Hz or so. So you really don't have to worry too much about it pushing the excursion of your drivers too much when the movie has 10 Hz content. I say that though because I am using a four 18" Avalanche IB, and have a fair amount of excursion to play with.
The only time I really use it is with older moves that are dead in LF end of things. You know, the ones were there is an explosion with lots of sound but no rumble.