Descartes

I was thinking back upon a discussion we had in class about which philosopher's views were most in line with our own, and I recall how many students discredited Descartes based on his principal of "cogito, ergo sum." However, this statement seems infallible to me. If you make the argument that this is just an assumption being made, then you are unable to assume anything at all in life, for if any assumption is true it is this one. Even by disputing the validity of Descartes statement, one is in a way reinforcing his belief. One has to think of an argument, and by this thinking it is quite evident that you exist in one form or the other. If you do not exist by thinking, then what would be the benefit of thinking of an argument to refute Descartes claim?