I didn't know that could be done. I've never even opened up the Accessibility panel. Actually, grayscale is not too bad. Everything is sharp and the contrast is high. However, I think I'll keep things in color anyway.
Alas, third-party apps—and of course websites—aren't made to be converted to grayscale. This can be easily seen by comparing custom-made monochrome icons in the menu bar to grayed icons in the dock and third-party toolbars (e.g. browser addons).