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Hmmm.. That's strange considering Steve Matteson (the type designer)[1] optimized the typeface for both mobile and web use. Perhaps the mobile browser you're using is rending it quite differently.

1: http://www.google.com/fonts/specimen/Open+Sans

Open Sans has plenty of weights to choose from, I don't think you should write off the whole font! http://www.google.com/fonts/specimen/Open+Sans

I'd love to know what browser / os you're using. I'm running a galaxy s3 in 'tablet mode' (lcd density set to 160 from 320) and the text renders nicely.