I have one on each row (three seats each row).
I don't know anything about the other shakers, I think you need to test each to really know which one is better ... I think that the size matters for installation, but it also helps with the power to move the seat
I'm happy with my Buttkickers ... I can feel the seat moving during the movies, and to shake the room I have my Subs ... a couple of weeks ago I got a demo DVD, and it has the scene from Nemo when the girl tap the tank ... What I surprise was for me when the room and seats were shaking ... it felted so real, like I was inside the tank ...
Like any speaker or sub, you need to set up the crossover properly or to your taste to have a good experience ... if you don't want your seat moving all the time, you can set the crossover of the buttkicker to respond to only the frequency you want (let's say less than 40, 30, 20, etc..) I have mine set at 80 and the volume is like a third of the dial ...