A real "Today I Learned" for a change: Occassionally you may get NPM shouting at you about access permissions. If you do, try running the following (Hat Tip to the ever knowledgeable Flavio Copes):

sudo chown -R $USER /usr/local/lib/node_modules

In my case, the above did not work, however, you should try running this on your user account .npm folder too:

sudo chown -R $USER /User/Username/.npm