I think mushrooms helps us realize that really, there is no truth... that there are only approximations to truth. Realization of the absurd, that existence has no inherent meaning, but we are all responsible for giving life our own individual meaning. The realization of the separation between what we know (science) what we don't know (metaphysics/theology) what we think we know (some hold the psychological need to know, so they make things up in their minds to claim certainty to) and the difference between them all.
The realization that, since there is no inherant meaning, since the world will not be here forever... why don't we all just shut the fuck up, get along, be nice to each other... and do our best to be happy, and help others do the same.
This is what mushrooms/acid does to me. Realization of the absurdity of existence.
Edit: (The joy, happiness and extreme feeling of freedom felt when one refuses to lie to oneself... to refuse to tell onself truths in the absence of it... and strives to seak the closest approximation to truth regardless of the of the pains of not knowing)