I've tried a few Card-Battle type apps over time, and CUE is by far one of the most entertaining AND legitimately informative game on CardsUniverseEverything store. There's a staggering amount of cards, all with factual interesting information about the card on the back, the artwork is really fantastic and nice to look at.
The variety of cards is also very impressive, there is a vast number of cards and the majority have unique effects, so no two cards are quite the same which leads to an almost limitless variety of decks that can be built.
My only critique, if any, would be to add a mode where you can have larger decks perhaps?
Also, i'd like to see the UI for building decks refined somehow, with the absolutely staggering amount of cards, it can be difficult to create a deck while remembering which cards you have, or which ones fit under 'X' category etc. Maybe some way to tell if a card can be effected by another, or like if they are in the same "family" of cards, since there is so many categories.
Overall fantastic game, no NEED to spend money to progress, you can certainly succeed without it, but should you choose to do so, i believe the market is fairly priced and you receive an adequate reward for doing so.
9.9/10
the ' .1 ' is only because the deck creation can be a bit overwhelming when you have a good number of cards, and only grows as you gain more cards.