Virtually all fonts listed in Google's directory render poorly on Windows in larger pixel sizes. This is caused by the lack of ClearType-specific hinting in these fonts, and it leads to the appearance of so-called "nipples" - odd pixels sticking out at the top and the bottom of glyphs and from thin horizontal strokes.
More bad news is that properly hinting fonts for ClearType is a relatively expensive process, and very few foundries do it even for commercial fonts. Unless Google goes beyond simply listing fonts and starts cutting cheques to polish them, these fonts will remain largely unusable for Windows' audiences.
At I/O they seemed to be talking about some automated hinting process to try and fix this. It won't be as good a something hand-hinted by a professional but it might be good enough for some uses.
FontSpring uses auto-hinting [1], which is a well-known approach and it is a step forward. It still though vastly inferior to manual hinting and tends to produce artifacts of other kinds. Also there appears to be no ClearType-specific version of it at the moment.
