Rick,
I've had a Buttkicker Gamer for something like 8 years now. It's awesome in Aces High 2 (
www.hitechcreations.com). Nothing being rattled out of your seat and having dental fillings loosened when the Thunderbolt's 8 .50s open up. It scares the crap out of the cats when it kicks in.
For FSX, not so much. I don't think the sounds for most planes are optimized to take advantage of it, although I haven't spent much time tweaking the Buttkicker's knobs to try to get a better crossover frequency or taking the volume knob to 11. I get a little bit of bumping while on the runway, maybe a light thump when the gear locks up or down, but barely anything from that big ol' radial on the Beaver let alone other engines.
I don't do much with first person shooters any more, but it certainly adds a new dimension to Team Fortress. You KNOW when a rocket explodes near you. I highly recommend the Buttkicker Gamer for that.