It's just a state of extreme bliss, plus (and distinct) an appreciation for all sorts of suddenly very beautiful things: music, landscapes, people, etc. Sound especially feels incredibly precise and 3-dimensional.

Long term, the effect is rather small. I didn't have any negative hangover as some people apparently do, but rather a sort of low-intensity glow that slowly diminishes. In terms of treating depression, I don't think it made much difference either way. But since I have been suffering for as long as I remember, it was actually the first experience of such strong feelings I now at least know what improvement would feel like.