I use the very basic:

    ffmpeg -i FILE -codec:v libvpx-vp9 -qscale:v 0 -codec:a libvorbis -qscale:a 0 FILE.webm

And end up with really good results out the other end. When I was rendering with pre-VP9 I needed a few more flags (stuff like crf) to maintain quality, but this works fine for most things. And has been really impressive on the filesize side of things.