Been using a ButtKicker for 10 years, and a custom thumper seat prior to that for about 10 years. The effect is that you become "one" with the kick, considered by most to be the heartbeat of the kit. For most drummers, it's crucial to hear that in the mix, perhaps more than any other drum or instrument on stage. Monitors alone can be very inefficient if you're not hip to how waveforms work. That means, a 15 or 18" speaker seems like a good idea for a kick monitor, except that the waveform doesn't develope for several feet. And that means the bass or guitar player standing 15 feet away will be knocked over, while the drummer hears & feels almost nothing. Some of the best physical monitors I ever had were a pair of 12s, which moved the air perfectly for where I sat, and didn't kill the other guys or interfere with FOH.
Anyway, the abiilty to detect what our drums are doing is crucial to playing them properly, in the same way that a good mix typically determines how well we play.
The ButtKicker - and please do not call it a shaker - is a piston housed in liquid, and operates from 5-200 cycles only. Perfect for feeling, and becoming kick. It's a very intimate sensation, you know excatly what your kick is doing.
Most users of such a device also have some kick in their monitor, just to get a more aural sensation, and not rely completely on the thump itself. The feeling is like the kick is shaking the riser... like it's really loud and punchy... but it's very internalized, just as our 'time' is.
A ButtKicker or thumper device might be overkill and more coordination than is worth it on small gigs. I don't use mine in town, and have never wished I had it on one of those gigs. But in a concert setting, I'd never want to be without it. I even carry a spare.
Bermuda
PS - ever been in an Imax theater? That low end is probably coming from ButtKickers.