For some perhaps the color of the text is the only complaint, but for me the font is also illegibly thin [1]. I got around it by disabling the web font, because just setting the font weight to 600 didn't fix the odd shapes [2].
The bold version of the font is also available as a separate font family (AvenirNextLTW01-Bold). It looks much more like a "normal" font weight and is incredibly readable [3].
I noticed that thin fonts are not so thin on high DPI screens. Example: that font is more readable on my 9" 2500x1600 (approx) tablet than on my 1080p 15" laptop. Maybe they are designing for retina Macs.
The bold version of the font is also available as a separate font family (AvenirNextLTW01-Bold). It looks much more like a "normal" font weight and is incredibly readable [3].
[1]: http://i.imgur.com/uVHXptR.png [2]: http://i.imgur.com/IXwR6EU.png [3]: http://i.imgur.com/xgz7EtR.png