I've been using Monaco for Powerline for quite a long while now. The main issue is that most of the builtin proportionally-spaced fonts have some glyphs which are difficult to distinguish. Monaco gets the job done but other people prefer slightly different glyph styles or want to make their own fonts, more power to them.
The lowercase 'a' is much better than Monaco.
The tilde and lowercase 't' seem harder to see than Monaco.
People see characters differently, so some things which make it harder to see for some are easier for others.
Make one want a website which generates a custom font based on each person choosing the most visible variant of each glyph.
Hack: http://screencast.com/t/nnGD3Wm1M7Z5
Monaco: http://screencast.com/t/65hJcI43XT7