Firstly, unlike so many "it was all in my head" plots, the other characters did not act exactly like themselves, only morphing into not-themselves symbols at the end of the episode (Star Trek always did that). Everything was off-kilter from the beginning, and it was only the intrinsic oddness of House that kept one from guessing what was going on (although once House himself figured out the first hallucination, I easily guessed the rest). But frankly, the plot was too odd for even House. (It says something about the writing that they could keep you going for as long as they did--a half hour in my case; yes, I am susceptible to well-written plots,and I didn't guess the Sixth Sense twist until the very end.) Chase was too much like House, Cuddy was too girlishly pleased when House could walk, Wilson was too analytical. No hospital would stick a patient in with a gunman (where are the police?) The "fake" patient was too weird (even for House; that was the beauty). The eyeball was too horror movie gothic. In retrospect, everything was wrong right from the beginning.
Even more beautiful was that it actually meant something. That is, it wasn't just House having hallucinations and then, bingo, we all wake up and oh, it wasn't real so who cares. The first thing that struck me as off-kilter, for instance, was the idea that House would tell the supposed wife about the supposed husband's affair. He never does that unless the affair and the disease really are linked. It struck a false note. But evidently, it is a possibility, a fear, that House himself imagined. It crystallized for him the argument at the end of the episode.
In other words, the guilt is real, and the apology was sincere. It expanded House's personality. When he apologized, he was, in effect, apologizing to himself (which is less corny than it sounds). The gunman's arguments (in effect, House's arguments to himself) weren't wrong. They were actually, typically, House--that we aren't kind for sentimental reasons but from a necessary lack of hubris. But House was also apologizing because the arguments wouldn't make any difference. "I know what's wrong," he says, and he goes down and rips the "fake" patient to shreds.
And the show retained that complexity up until the end. When House asks for Cuddy's (not real) treatment, he is asking for his leg back. But in the hallucination, he was willing to sacrifice the leg to have his mind back. And both desires are true. It's the essence of House that he wants two things that he believes are incompatible. It is what makes the show work so well in the long run.