Fiendishly addictive. (Pun not intended, but enjoyed)

It's one of those games a bit like Farmville and Candy Crush where... I don't know the ins and outs of it, but I'm pretty sure it's designed with the assistance of psychologists for maximum addiction. It pushes all the right "buttons" in your brain that give you satisfaction from filling up bars, getting maximum stars etc and gently nudges the goalpost as you go along. A few hallmarks of these types of games are highly ornate interfaces, large colourful fonts, a character that "talks" to you, and an air of congratulation just for opening it. It all contributes to the effect.

The general idea of course is that because it's stamina based and mythril is rare you'll keep feeding it potentially substantial amounts of money to play.
It also has the gambling addiction side of things down with the very expensive relic draws.

At least it doesn't force you to pay a single dime, you just have to be careful as it's basically subsidized by addicts so you should try to make sure you are not the addict.

I'm somewhere in the middle - I have the willpower and enjoy the game, so I occasionally toss them £0.79 for a continue, to help support it.
It does get a bit "grindy" though like with this sector 7 event over and over and over farming magicite.
And I'm only a short way through the main game as I've not played V yet and don't want to spoil it (playing the first V room is a requirement to unlock anything else)