I had a pop-up that had a cassette toilet much like Roy pictured. Honestly, we never used it for the poops, just pee.. I don't think I ever sat on it.
My wife and daughter, however, were frequent flyers. I never heard complaints.
As for dumping it, we just use the dump stations at the campgrounds that we were at. The external access to the cassette made it easy work to pull and put it into the back of our SUV. Plenty of water and we were off. I did use a fair amount of orange liquid from Walmart to keep down smells.
Knowing what I now know- I'd likely use it for poop, but we're long past our pop-up days.