Definitely very important to me
Personally I use a patched version of 13pt Menlo Regular. It's patched to add symbols for powerline.
I am using Menlo as well. But instead of powerline I switch to https://github.com/maciakl/vim-neatstatus.
This reminded me that I never reinstalled Source Code Pro on my new machine. It's worth a shot since it's really clear at small font sizes.
 https://github.com/adobe/source-code-pro http://www.fontsquirrel.com/fonts/source-code-pro
Personally, I'm a big fan of Monaco 12pt. There's a powerline version here https://gist.github.com/baopham/1838072
I use and love Inconsolata-dz 11pt (or 12pt). Powerline font can be found here