I’ve been seeing a lot of sites using Open Sans for their body text lately.
And it’s making my eyes bleed.
The problem isn’t Open Sans—it’s a great (and free) web font—it’s that it’s too often being rendered with a way too small font-size and a way too light font-weight.
As that Typekit screenshot tries to demonstrate, I don’t really think you can use font-weight: normal with any size below 16px without losing some serious readability to the font’s narrow nature.
However, use the book variant to widen up the characters a bit (via font-weight: 300 with Google Web Fonts) and maybe keep that size at something higher than font-size: 11px (Google Web Store, I’m looking at you) then in my books, you’re swimming in delicious font gravy, baby.