I ended up having a really great time with this overall. I was around 50 hours when I hit credits the other night, but there are still things to do. I'm not super interested in completing the bestiary any further, or doing random quests for post-game, but this is probably a game I'll enjoy another playthrough with in a couple of years.
This is a bit long, because I broke the review down into overall mechanics and features, and then into my thoughts on various aspects.
I first played on launch day, but the game was crashing and lagging constantly, so I ended up putting it down for a few months until an update went out.
There are a lot of things to love about this game, from the more status effect-focused battles, to the field abilities your character can use, to the flexibility of the teams you build.
Overview/Mechanics:
The basic premise is that people have been transported from all different worlds (or at least different points in time from Earth) and ended up on this one, New Wirral. People use cassette tapes to record the wild monsters on the island, and then transform into those beasts to battle. There are 12 Ranger Captains around the island (think Gym Leaders) who you must defeat in order to become a Ranger yourself. That is one of two main quest lines that span the entire game, the other being defeating some really powerful monsters in order to find a way back home (if there is one).
You are in double battles for 99% of the game, where you and a partner can both transform and fight. Your human characters have a separate HP bar from your beasts as well, and if your humans get KOd, they cannot be revived in battle. (Humans get hit while recording or if a multi-hit attack KOs the beast and then hits the human before they transform again.) There is also a fusion mechanic, in which your two beasts merge into one and get the typing of both beasts. There are nearly 130 total beasts, and fusions of the same two beasts are also a thing, which means there are a crazy number of fusions to see.
There are fourteen types that your beasts can have: Beast, Air, Earth, Fire, Ice, Lightning, Plant, Water, Astral, Poison, Metal, Plastic, Glass, and Glitter. Every beast has a base type, and then there are "bootlegs" of them for every other type. Think of bootlegs like shinies that change the beast's type as well as its coloration. Some attacks can also place temporary "coatings" on a beast, which will change their type for the duration of the status effect.
The moves of a beast are determined by which stickers you place on their tape. While each beast has a different number of available sticker slots, all stickers are removable and reusable. Most stickers have a "blank" type, so the type changes based on the type of the tape they're placed on. ("Smack" placed on a Plant beast will do Plant damage, or Fire if placed on a Fire beast, etc.) The only real limitation is whether a sticker is compatible with a specific beast, and the number of sticker slots that your beast has. Otherwise, you can pull them off and rearrange them to your heart's content. It is encouraged to catch multiples of beasts so you can scavenge the stickers they start with. (Edit to add: Moves use AP. Your beasts get 2 AP at the start of every turn, unless a status effect is boosting/lowering that. Every beast starts with either Spit or Smack, which costs 0 AP, so you can still attack if you're out.)
Beasts can be remastered (evolved) once they hit 5 stars. Essentially, your human characters are the ones with an actual level. Beasts gain experience as well, but it raises their star level instead and nets them new stickers and sometimes more sticker slots. Once they hit 5 stars, they can be remastered the next time you rest (if they have other forms). There are some beasts that don't remaster, but most have two or three forms. There are also some that have branching remaster paths, like the two starter beasts. They each have two different second and third stage forms. Some are just a matter of picking the one you want during the remaster process, while others require specific stickers. Also, beasts continue to level even after they hit five stars. They don't earn more stars, but they do continue to get more and more powerful stickers. Your humans' levels are what determine your overall "strength" though, so you can basically swap your team around as often as you like without it really being detrimental to your team's strength, even though the beasts do have their own stats.
For recording new beasts, one partner comes out of beast transformation to record, while the other continues attacking. The recording takes one turn, and anything that does damage or causes new status effects on the target raises the chance of recording. If that beast does damage to your recording character, it will drop the percentage.
There are field abilities that your character gains upon recording some specific beasts throughout the map. This is one way that you're limited from going absolutely anywhere at any time, so even though you're free to explore in any direction, you're generally not just stumbling into an area where you're gonna get wrecked.
Thoughts:
I enjoyed the combat. While the battles were sometimes frustrating, they did present a usually-welcome challenge that isn't found in Pokémon or other Pokémon-like games. I really like the transformation aspect, as well as the focus on causing status effects based on how certain elements interact. Sometimes it's simple, and sometimes it can bury you in the strategies. As someone who doesn't like when battles drag out forever, this can be a negative point. Selecting an attack shows the effect it will have on the opponent beasts, which is helpful, but a single turn could go as follows: You use a Fire attack on a Plastic beast, which turns it into a Poison type. Poison attacks fuel Fire types and boost their AP, so you've given yourself an advantage. Say your other beast is a Lightning type, which will cause a conductive effect on the Poison enemy, which can then hurt it every turn, as well as the burn it likely has. But maybe the other opponent beast is Astral, which would get boosted by attacks from your Fire beast, and will take a lot of hits to go down if you don't use "pollution" types against it. Essentially, you can build up a shitton of negative effects on an opponent, but it's just as easy for them to slam you with contact damage, a burn, poison, lowered accuracy (while they boost their own evasion, and you basically NEVER EVER land hits when either is in effect, which is super annoying especially if they're cycling between lowering your accuracy and then boosting their evasion), and leech your HP, all in one turn. In short, a simple wild encounter can drag out for a long time depending on the status effects in place.
Exploring the map was a good bit of fun, especially with all of the field abilities unlocked. Beasts are visible on the map, so there aren't any random encounters.
I liked the story. There's not a tonnnn of depth, but it's interesting and was implemented well. I also liked all of the partners. They're all romanceable (except the dog, at least I think). Venturing out and gaining more relationship stars boosts your Fusion parameters with them. I mostly just stuck with Felix once I unlocked Fusion with him. Eugene kind of gets ripped off because the only way to unlock Fusion with him is to complete his quest, which requires visiting locations all over the map, even though you meet him early. I didn't get it until later in my playthrough, so I barely partnered with him.
THE MUSIC IS GREAT.
I didn't get into them, but there are lots of other things to do, like battle rogue fusions around the map, puzzles, and various quests for the townsfolk. After you beat the main Ranger quest, you unlock the notice board, but the second set of quests on that requires you to finish the other mainline quest. So basically all of the quests available there are post-game.
The team I spent the most time with/took into the final battle:

So many spooky bois. First one is the final form of my starter. Far right ended up being my absolutely favorite tape line in the game. (I had a Lightning bootleg version of Southpaw--the second beast here--from a promotion code, so mine actually had yellow instead of the maroon.) Types from left to right are Astral, Lightning, Fire, Poison, Ice, Plant. There were some insanely cool fusions from combinations with Apocrowlypse (Poison) and Hedgeherne (Plant).
There are a lot of fun references to the real world, punny names for beasts, and thought-provoking dialogue about life, goals, and acceptance. I definitely recommend to anyone who enjoys a good monster collector.