Kde is more of a resource hog, I can tell you that much.
Stable? It's Linux, so it's not too bad at all
It's worked on pretty heavily and has a better usability, I feel, than Gnome. You'd just have to use them both to figure it out. I like other window managers personally since they're faster and take up less room, but that's what's great about linux.. you have a lot of choices.