I'm somewhat surprised more people don't scape conventional bathfans for a central unit. The reason for this is 2 reasons 1/A study done a few years back (Tryed to find it in my files but couldn't) showing that the majority of such units only work @ 35% - 50% of their rating. This being caused partly by poor installation (flex duct in long runs; flex duct w/ sags, allowing condensation to collect @ plug; Lack of make-up air. A central unit has a more power (Grunt!), @ is less vulnerable. 2/ Biggest reason bathfans are not used is the sound, even the quiet ones. W/ a central unit, the motor is usually mount in the utility room (the quiet unit is one that runs someplace else).
I have a single unit (DEC Int.'s Quietvent)venting 3 bathrooms & the kitchen area. Works great! In fact, in the bathrooms it is suggested to use a timer switch because you forget it's on. In the main part of the house I have it connected to a dehumidistat so that it automatically turns on when the whole house get's too humid. True, on a retrofit installation it may be more difficult (esecially if your basement is finished), but it's relatively no more expensive ($300 - $500) and a lot more efficient!