Unity doesn't even work all that well on full-powered desktops in some cases. Each release seems to come with new features in Unity but you never know if the desktop will become more or less responsive each time. On my machines I haven't found any to run as well as 12.04 yet (12.10, 13.04) and since it's an LTS it'll stay for a bit. But I'm running it with Cinnamon as the desktop environment because it's quite a bit snappier than Unity and I am no longer a fan of Gnome in any way, shape or form. I used to love it, but now it's Cinnamon or KDE for me.
So to answer the question, you really need to try it out on your hardware to know if it's good on it or not. It changes so much in performance each release. But, I can say it's dog slow on my Acer C7 Chromebook with Celeron processor and 2GB RAM, which is probably similar in specs to a netbook.