Ex nihilo nihil fit I have problems with cogito ergo sum; both the means by which Descartes arrives at this conclusion, and its implications. Firstly, its radical skepticism that suffers from Occam's Razor. To doubt that the external world, and even his physical body exist, because perhaps, a demon may be deceiving him is unreasonable, not least because - it implies a much more complex explanation than the apparent reality; and as Occam asserts, "the simplest adequate explanation is the best."
Having posited unreasonable premises, Descartes cannot proceed toward a reasonable conclusion. Clearly, Descartes does have a physical body, and the external world does exist, so cogito ergo sum does not follow; or, it invokes the unreasonable conditions of its birth in every moment.
This leads to my second problem; that cogito ergo sum is invoked free of this unreasonable context in support of subjectivist philosophies - as if, in reality, the only thing of which we can be certain is our subjective experience. It seems to me, had Descartes thrust his hand into the fire - rather than a ball of wax, he would have become suddenly and painfully aware of the existence of his physical body and an objective reality; an awareness that would be prior to "cogito" in its undeniable urgency.