You can be "nice" and have people like you
or you can get things done.
There's always some sort of balance between the two, but the most efficient way of doing things is to put any "niceness" aside.
EDIT: On another thought, manipulation and kindness are two different things entirely. It all depends on who you're working with and/or against and how to best handle them. Be "nice" and get them to do what you want or be straightforward or even "menacing" in a way to get them to work hard.
VI VERI VENIVERSUM VIVUS VIVCI
"By the power of truth, I, while living, have conquered the universe."