Some of the commenters on this thread seem to be viewing the change caused by psilocylbin as foreign or hostile in some way. Maybe I'm approaching it from a different perspective. My emotional tendencies fluctuate from hour to hour, day to day, and month to month. Just eating a good meal brings me from grouchy and irritable, to happy and creative. I don't feel like my identity is lost during any of these transformations.

Going from sad to happy doesn't change my utility function/preference ordering, my beliefs about the world, or my level of rationality. It just makes me able to enjoy all these things more fully. So if I were assured that the psilocybin would just change my happiness set point it would seem like a pretty good deal.

Applying the wirehead argument here seems a case of lost purposes. The problem with wireheading isn't that you're happier than you "should" be, it's that you've lost the chance to get all of those things in your utility function that aren't just happiness, because you're too busy blissing out. I see no reason to believe the psilocybin takers are at even the slightest risk of that.

A question to anyone who wouldn't take the psilocybin: if you had depression, would you take Prozac (assume for this hypothetical that Prozac is proven to work)? Do you think the answer to this question should always be the same as the psilocybin question?