Okay. Well there's a few things. First, there's a debate over whether serif fonts are more readable than sans-serif, but I think that's all subjective.
Assuming you think helvetica is a good font for website content (a lot of people don't think it is), helvetica nueue (one of its more-expensive variants) was made as a font for featured content (titles, logos, etc) and was never meant to be readable at small sizes.
Besides just looking strange, the eye can't read it as fast.
Definitely more readable with readability, but that seems to be mostly a matter of font size. If I reduce the font size of the output from readability with C-- in firefox, I don't see a big difference in readability.
