Re: Composting Toilets vs. Holding Tank
For what it is worth, I switched to a Nature's Head after not paying attention and overfilling my old holding tank with rather unpleasant and unsanitary results. It scarred me and I swore "Never Again!"
It took a long while to pull out the old system. There is a lot of stuff that goes along with it, jammed into all sorts of unlikely spaces. The new unit is self contained, is simple, with practically nothing to break (the fan is probably the weakest link), and just works. The girlfriend was probably a little skeptical at first (I think she spent a couple days holding "it"); but is now fully onboard (provided I do the pee bottle dumping). The bonus is I now have the room for a new 20 gallon water tank, which will happily double my capacity. Score!
[The waste disposal issue is still a concern, and I'm not feeling terribly guilty at this point, especially since dumping once, maybe twice a year might be enough]