Peggy is right; if not installed properly they can clog. But contrary to the flavor of this thread, there are a whole lot of folks that are happy with them. I have tested larger vents, treatment chemicals, and vent filters, and they can all work. The best choice... depends.
In my case the vent was very near a critical hatch
, and a vent filter provided a perfect solution.
a. I have a cat and it doesn't heel much.
b. I installed it in a high location and provided a trap.
c. I still use a nitrate-based chemical (Camco Ultra or Forespar Refresh, both excellent). These reduce the odors in the tank, reducing permiation and possible odors from flow-back (we spray the chemical in the bowl before leaving--work great).
The canisters last ~ 2-3 years and can be refilled in 5 minutes for ~ $5.
Sail Delmarva: Holding Tank Odors vs. Carbon Vent Filter