We've had ours (Nature's Head) for several years, and it has performed well. My neighbour has the Airhead, which he also likes. All marketing hoo haa aside, they are both good units that do what they are supposed to do. Some of the biased stuff said above is just not true, by the way. Advantages or disadvantages of either are minimal.
Overall, it's a great way to go. We eliminated the faint but always present odor we had with our (expensive!) holding tank system. This thing does not stink - at all. This alone is reason enough to have it, for us. You put toilet paper in it, no problem.
Pee is sterile, not a health risk and easy to get rid of. However, the CG tells me that while it is legal to pee overboard, it's not legal to pee in a bottle and pull it over board (they laughed with me on this, and saw the absurdity of it). So be sensible, and discreet.
The compost, containing recent poops, gets dumped in a toilet in a marine park or ashore. With two of us full time, this had to be done every six weeks. Or, if we are not travelling we let it fully compost in a bin at home, for a few months, then put it on the plants.
We gained a huge space when we removed the holding tank. And it felt great getting rid of all those hoses and fittings, and a pump - all of which stank, clearly, when removed. It all stinks. If we are honest, most of us have to admit with holding tank systems we get the odd whiff, especially if the boat has been closed up for a couple of weeks. These composting toilets just do not stink. No, they are not unsanitary. They are suitable for up to 2 full time users, more part time. I would never even consider going back. It's the best invention since self steering.
We had great service from this guy, but there are others. http://sustainable-solutions.info/