Depends on how many fans you have, and how big they are. I don't know the maximum current that can go through the rheostat. It's working fine with my chunky 120mm fan no problem, but I dunno how it'd cope with more than one. Also, you'll probably want to control each fan individually, I found there's a certain RPM for each different type/size of fan when it's running fast enough but quietly - a 'sweet spot' if you will. You'll be wanting ot set the fans to this sort of speed. For my setup I use zalman fanmate's for 3 out of 4 of the fans: the CPU (big 120mm over zalman heatsink), graphics card (zalman quiet jobbie) and the exhaust fan (just an 80mm LED fan). These always run at a constant speed at this sweet spot that I mentionned. The musketeer is wired to another 120mm intake fan at the front of the case, normally it's set to minimum RPM and still sucks in a really nice volume of air, but if the computer is getting a little warm (hot day or whatever) I can crank it up and the fan goes a bit crazy and shoves massive amounts of air into the case normally dropping the temperature by about 5 or 10C. I use a lot of passive, silent cooling in my machine so the ambient air temp is usually quite high.
I'm rambling.... Basically, get either small rheostats for the fans that aren't going to be changed or buy a proper fan controller for the front of your case (there's some nice ones with controls for 4 fans in the same aluminium/blue LED fashion as the musketeer). The musketeer is really only for a sinlge fan.