I have made some comparative tests with OpenSans and ttfautohint 0.8. with WinGDI Cleartype and Greyscale.
Tested original font (OpenSans regular, Light, Extra Bold), plus - unhinted, ttfautohint with 'taller xheight', and regular ttfautohint versions.
The most significant issue remaining is the rendering at small text sizes with bolder weights, e.g. ttfautohinted versions can't yet emulate the finesse of a human manually hinting chars such as 'a', 'e', 'g' etc with such few pixels to play with. However, to my mind in the medium size text ranges i can see characters rendered a bit 'better' by ttfautohint ;)
Another gripe i still have is that ttfautohint tends to stretch the xheight by 1 pixel at particular small size instances. In my screenshots this happens noticeably at 12px, where the manually hinted x-height retains better proportion.
Suggestions for alternative fonts to test please. Manually hinted to 'industry standard' of course.