If you don't know Japanese, it might be worth waiting for this to inevitably hit the English storefronts. Many cards have complex effects that you likely won't figure out without reading.
There are five classes (called Themes) you unlock by progressing the story, and each one has a different playstyle. From self damage and chaining attacks together, to sacrificing other cards to deal heavier damage or trigger extra effects, there's nice variety to choose from. You're often forced to play specific Themes, so you'd better get used to using them all.
Compared to Slay the Spire, Devilaby gives you hardly any healing or shields, so the main mechanic you have to work with is switching your party order. The party member in front takes the hit (and can gain shield normally), while the member in the back heals. Longer battles become about measuring how much you can spread out incoming damage, and how much you can put that back slot healing to work.
The other unique mechanic is Sheep, a replacement for items/potions. Sheep are treated as cost 0 cards in your hand once used, and some even cycle through your deck like any normal card. Because they're weaker than the usual Potions of other games, you often need to know when to use them early to stop a bad situation later.
There is a gacha in place, but it uses the easily accumulated Demon Coins to unlock new cards and artifacts per Theme, much like class mastery/levels do in other games of the genre. There is a more premium currency available as well, which can be used to manipulate rates that card rarities appear during runs, among other very Kemco bonuses. You can earn this currency endlessly via dungeons as well.
The plot isn't much to write home about, but it knows to not take things too seriously and keeps itself as a light hearted story about dumb demons doing dumb demon things. The world is filled with a fun variety of demons and monsters to keep things interesting, even if the enemy variety can be a bit lacking.
My main complaint, however, is the absurd frequency of advertisements. If you're at all familiar with this genre, you're likely moving very fast. There are 2 ads per floor, so you're going to be watching these every 3 minutes or less in some dungeons. It's a LOT.
tl;dr A solid game in its genre, with some kind of unique mechanics in Switching to keep things interesting.