This is not an important criticism; it is ad hominem in its purest form.
Prediction: Given the right incentive and five minutes to think Eliezer would be able to give an example of a criticism that is a more pure form of fallacious ad hominem. I am only slightly less confident that a randomly selected 15 year old student, allowing the '5 minutes' to include an explanation of what ad hominem means if necessary.
I just watched Transcendent Man about the singularity and Ray Kurzweil in particular. It's well-made, full-length, and includes the most popular criticisms of Kurzweil: that his prediction timeframes are driven by his own hope for immortality, that the timescale of his other predictions are too optimistic, that his predictions about the social outcomes of revolutionary technology are naively optimistic, and so on. Ben Goertzel and others get much face time.
You can rent or buy it on iTunes.