From page 12 of "The Power of Now," by Eckhart Tolle:
"The philosopher Descartes believed that he had found the most fundamental truth when he made his famous statement: "I think, therefore I am." He had, in fact, given expression to the most basic error: to equate thinking with Being and identity with thinking."
"The compulsive thinker, which means almost everyone, lives in a state of apparent separateness, in an insanely complex world of continuous problems and conflict, a world that reflects the ever-increasing fragmentation of the mind."
His message, in my view, is not so much being down on thinking per se (he does say the mind is a great tool once you've learned to use it properly) but to not let thinking and mind identification cheat us out of the only thing we really have - the wonderful present moment.
