What nobody ever mentions when talking about these portable composting toilets, is that if you take a crap in it on Tuesday, and then empty the solids container on Wednesday - what you are emptying is NOT compost...it's feces.
It CANNOT be spread as fertilizer because it has not had the time (or temperature) for the composting process to destroy the bacteria and pathogens.
Composting takes time. If there is also temperature, then the time can be reduced, but even with temperature, it's not a fast process. That's my problem with these portable composting toilets - I can see using one for a few months, and then letting it sit for a few months for the composting process to run its course, then emptying it. But empty it too soon after using it, and what you are disposing of is NOT safely composted human waste:
http://weblife.org/humanure/chapter7_19.html
I'm also pretty sure it's not legal to just toss it in a handy dumpster either. Most states allow the solids from a composting toilet to be thrown away only if A) it's actually compost, and B) the trash is destined for a "sanitary landfill":
http://weblife.org/humanure/appendix3.html
http://web.mit.edu/urbanupgrading/urbanenvironment/sectors/solid-waste-landfills.html
Uncomposted (or partially composted) human waste is sewage and so the normal sewage rules apply.