We used a Nature's Head on our sailboat when living aboard full time. We emptied the solids every 2 - 3 weeks, no smell, much better than the usual heads on a boat. We emptied the urine bottle every 2-3 days, always adding water to start and when I dumped it at the base of trees-- or anywhere in the drip ring of a tree. At least 1:1 urine to water. It's great fertilizer.
We bought it new, were very happy with the Nature's Head. We considered the Separett but it uses plastic bags-- they're compostable, but it's a continuous expense that we didn't want.
When we build a house on dry land, we will likely use wheelie bins under a toilet or a vault in the ground. The vault toilets at Ecovilla Gaia, where I did my PDC, went years without needing to be emptied (with lots of users), because they used worms in the vault. It takes some explaining, but it is a great system.