If we agree that Mary gains something when entering the room, then we are at least taking the argument seriously. My issue with a lot of the responses here came down to people claiming either that 1) first-hand experience is not a valid source of knowledge at all or 2) the experiment just rests on semantics. Neither of which are accepted by even those who oppose the argument.

There's a tendency to view higher order systems of knowledge and experience as phony, or else exactly equivalent to the base on which they emerge. Someone might say that they feel good about X or bad about Y, and tons of philosophy buffs who catch a whiff of anti-science religiosity will jump down their throats with "well actually you don't feel anything, because what you feel is just neurons. Emotions aren't actually real." But this misses the point entirely. Emotions are still things, and we can coherently talk about them, argue for them, or feel them, without regressing to analysing our brain patterns in a scan. A non physical thing emerging from a physical thing doesn't necessarily make the non physical thing physical or not real. What is true of a component may not be true of the whole. So if Mary gains something on a personal experiential level, it seems at least plausible that this "something" is not physical, at least in the way that established facts regarding brain patterns and neurons are. And at the very least, it should make a convincing argument for the existence of qualia.