Due to the LOW amount of flush water because it is a direct deposit device with the poop NOT traveling to a tank it is BIG at 9.5 gallons
We are pretty good about timing are deposits and while we did have it pumped twice in 2011 we cold have gotten by with once
Interesting device. Probably a good choice for smaller boats who don't want to go the composting route. An EXCELLENT device when matched to a maceratior/overboard discharge for Canadian boats. Most canadian boats of any vintage are direct-overboard flush only, which in most areas is totally reasonable form an environmental point of view, but a unit like this would allow you to keep the stuff out of the water while travelling by the oyster farms of Desolation Sound, or in the crowded low-flow harbors and anchorages.
As long as it has some flush water it might smell less than a porta-pottie, but on the other hand, it'll be an anerobic tank and every time you make a deposit, I would expect smell unless you use chemical warfare.
As for the previous comments about there being no way to dispose of composing toilet waste, the answer is to put it in the trash. I like Knothead's comparison to diapers in the trash. There was also some hysteria about the dangers if someone's dog got into the trash, or the poor trash handler etc, and to those I would say that trash is rife with bacteria. That raw turkey you threw out 3 days ago is now crawling with salmonella and shigella. The cat litter is full of e-coli and even toxoplasmosis
and yet sanitation workers and those who rifle through trash seem to do okay....
The Nature's head has a neat design feature whereby a yard-waste trash bag fits snugly over the lower part of the toilet. Turn it over and it all goes into the bag. No touching of poo required. Though I still wash my hands afterwards for good measure.
PS Knothead, I can't speak much to the question of development of immune systems as a child because there isn't enough good evidence one way or the other that infections "strengthen" the immune system. What I can tell you is that the immune system is largely developed before adolescence and doesn't change much as an adult. Many of us who go into healthcare fields suddenly find that we never get sick anymore and some (usually not the providers because we're supposed to know better) attribute it to "a stronger immune system" when really the evidence shows it's because we wash our hands obsessively.
For further reading related to your question, check out "The Hygiene Hypothesis." A mature, but unproven, hypothesis that allergic diseases specifically are caused by the lack of parasite exposure in the developed world. When reading about this topic or ANYTHING MEDICAL please PLEASE skip right over Wikipedia. ANY website is better than they are on medical topics.