Crystal clear on my Firefox.

Lesson for everyone. Beware! Cool, funky, gorgeous fonts on your machine may be downright unreadable for others.

Stupid question - as a temporary measure, have you tried increasing the font size? Something like Ctrl-+ should do that.

What are you using beyond Chrome? The Open Sans web fonts render much better on my Mac with Chrome Version 30.0.1588.0 dev.

Not that I'm a big fan of the font (too close to Arial, especially the 'R' for my antiquated tastes), but I would say it's a well-rendered font of a typeface I do not like. :-)