Are Successful People Nice? - Art Markman - Harvard Business Review
Tina Miller, MA,CFLE stashed this in success
Career success also involves being critical. While some managers may want to surround themselves with people who obediently agree, most want those who will find the flaws in a plan before it is implemented. Less agreeable people are prone to give this kind of criticism.
In other words, it is the more critical people who are more successful, not the nice ones.
The nice ones are often too passive. Success requires being assertive.
I don't think being nice and being willing to critique are mutually exclusive (unless one is defining nice as being agreeable, which I don't think would be accurate). I think success would be a combination of several qualities -- including kindness mixed with a willingness to critique when necessary.