This
game
is
AMAZING.

The mechanics are well-oiled (especially after the last updates; now, it flows smoothly). The OST is simply beautiful. The graphics create a unique atmosphere: the gameworld is dark-toned but not lugubrious; you feel lonely, but never properly desperate, and the scenarios are always enjoyable (even after countless hours of replay). Also, the storyline of sorts is gradually revealed via item logs and monster logs you acquire after some time playing - the game starts in media res. You don't need to know what's going on to enjoy it, but it surely adds to the flavor. I particularly like this form of storytelling: a very good game becomes more and more fascinating once you start realizing (well, perhaps not exactly) what's going on after some exploration. It reminds me of Diablo II in this point.

Most important, though, is that it is PURE FUN. Very challenging and very rewarding, it leaves me wanting more every time I play. Actually, I sort of developed an emotional attachment to this game; when I started playing it, I was somewhat distant from videogames (after 24 years of being a total videogame nerd), not really willing to play too much. This game (along with Papers, Please and some others) helped me "feel the magic" again, sorta like falling in love once more with your boyfriend/girlfriend after a period of strain and doubts.

tl;dr: FUN, rewarding, beautiful, challenging, cool, buy the hell out of it