Materialists generally think that conscious experience somehow emerges from mindless matter. This is very different from saying that conscious experience doesn't exist.
Even if we assume for the sake of argument that the concept of emergence makes sense when applied to consciousness, I think there is a gedanken experiment that is worth thinking about. As with all gedanken experiments you have to accept that it is somewhat far fetched, but not in any essential way (or argue the contrary).
So we imagine a person with his emerged mind thinking thoughts that make him unhappy but don't require any new input or generate any output. Perhaps he is thinking about a failed love affair. This man then has his brain scanned destructively - so he is killed in an instant, but all the essential data about his mind (maybe part of the brain, or all the brain, or even his whole body) is transferred into a computer equipped with a program to simulate how his mind would evolve from moment to moment.
From the POV of a materialist, the computer is equivalent to the original brain, so as it executes, it should feel all the pain that the man would have felt if his ruminations had not been cut short by the scanner!
So far so good, but because the program can be run over and over again, one has to ask if this would cause repeated bouts of love sick angst!
Even worse (from the POV of the materialist) the program is really equivalent to a theorem - running the program consists of a sequence of logical steps that can only have one outcome! This means that like a theorem, you don't actually have to run the program - it is true anyway. The suffering has been transferred to the platonic world of theorems!
To be fair, though, atheists and materialists do say some silly things about mind and consciousness. I believe the main reason for this is that they are often engaged in stuck-on-stupid debates with theists, and they are afraid that if they admit that consciousness, free-will and morality are mysterious and baffling, then that will make life very difficult for them in the context of the debate.
I think if you gather up all the silly things they say, they amount to a set of evasive tactics to avoid a conclusion that they don't want to reach.
I wish more atheists would follow McGinn and just admit that science will probably never be able to deal with 1st-person subjective experience. What's wrong with just living with the mystery of consciousness?
Well fine in a sense - if they didn't try to pretend that they 'know' the answer is to be found in materialism!
Remember that in 100 years time, the above gedanken experiment might actually be possible - leaving aside ethical considerations!
David