In a sentence: This game plays like the classic Pre-ATB Final Fantasy games married Pokemon, and their wedding was officated by Magic: The Gathering.

I mean this in all positive and negative connotations.

It has a classic party vs. party battle system of Final fantasy, The party-member acquisition of a more-or-less forgiving Pokemon, depending on your viewpoint (More forgiving, because you don't need luck, less forgiving, because some of the rarer monster require you to beat them 20 times before getting them.)

The real cherry on the cake is the traits each monster has that makes the MTG comparison come into play. I hope you love rules lawyering as much as I do.

Every monster has their own unique trait, some of which can be found in item form and attached to equipables. Some of them are simple such as "This Monster does 75% more damage to enemies with X status effect," all the way up to "At the start of combat, the first 3 non-ethereal (temporary) spell gems on each party member has a 20% chance of being cast." There are conditions that are even more specific and interesting, and you can have a lot of these active at once when you unlock fusion.

It's great if you have a programming mindset. There's a fully functioning marco system with conditional statments, as well. Enjoy my fellow autists for logical comparitors.

Pokemon Infinite Fusion meets Final Fantasy meets Digimon World DS meets the random number generator from my nightmares meets Ritalin.
This game is fun. Simple in premise yet infinite in possibility (and yes there is a near infinite amount of playability with scaling). With exactly (at time of review) 1201 creatures each with a unique trait, dozens of character classes, hundreds of spells, and thousands of crafting materials and decorations to hoard, creating a team build is less about "Can I make this work?" and more an exercise in "How broken can I make this?".
And let me tell you, boy does it milk that sweet sweet serotonin drip.
A warning, this game is not nearly as popular as it deserves. Information is scant with the esoteric intricacies of team mechanics only rarely being illuminated by sage masters on internet forums. Be prepared to do most of the trial and error on your own. But, that's the point isn't it?
At the time of writing I'm not a 10th of the way through the achievements and sitting at a meager floor 110 out of, theoretically infinite but, 460 to reach final end content. The campaign is simple and really only serves to trickle feed mechanics to the player with the real goal of the game being a test of how far you can take your team comp against higher and higher odds.
Worth every penny, on sale or off. Always and forever will be a go to recommendation in bang for buck content.