I think they are excellent, but need to be treated with respect.
Most important of all, young children do not go near them. For this reason I'm not keen on the newer sort with 'air switches', as I understand that they operate automatically. I prefer ones with a switch you have to operate deliberately.
Best to have the wdu in a separate sink, to minimise the chance of teaspoons (or Star Troopers) accidentally going down, but not essential.
Most of our green waste goes into the compost, but if it doesn't then at least it doesn't go into landfill. Similarly, only things that dont grind well, like large bones or raw chicken skin, go to landfill.
Our sink drains perfectly well for normal use. The only time that The wdu has any effect on the draining is when both taps are on at full flow with the edu not operating. But the only time that happens is for the second between switching the wdu off, and turning the taps off.