As a spin-off Anode Heart: Layer Null can be played without any knowledge of the first game, so if you are at all interested in singleplayer collectible card game campaigns you can and should jump right in.

There is a lot of strategic depth to this card game. A barrier is raised during the second phase which prevents the opponent from using cards and effects that target your units. This allows you to safely summon them during this phase without fear of getting hit by spell damage and denies energy gain from Fire spells. However, most units receive bonus stats when transitioning from the first to the second phase and the player with the most energy generated gets to draw an extra card once the second phase starts, so there is incentive to play during that first phase. A phase ends when either player passes after each has had a turn, so you also have the option of not playing cards while in the lead to force an early battle phase before the opponent can bolster their defenses.

There are a ton of mechanics to build your deck around as well. You can specialize in big stats, removal, milling, board lock, sticky units, summon from deck/graveyard, hand disruption, board disruption, expensive direct damage spells, and all sorts of other things. For most of the game I used a deck that outright wins at the end of the battle phase when you acquire 4 specific types of energy. When facing decks adept at slowing that down I often used a deck that swaps your units with enemies to give my opponent self-damaging effects. You will likely find your preferred style of play represented here, though it may take a while to unlock it. You will also have to face decks of all kinds along the way, thus my playtime reaching over 20 hours despite sticking to a small number of decks.

I personally did not get all that invested in the story, but it does seem decent enough. The protagonist is a player in a game and there is a lot of lore related to the game-within-a-game. Events related to the main plot play out with an appropriately serious tone but then some of the minor NPCs have delightfully silly dialogue. It's worth mentioning that the game is not currently complete, but it's very nearly there. The story involves collecting 12 stars across various servers and all 12 are available. It's just the last bit with the final confrontation(s) that isn't finished yet.
[Update] The story is now complete. What's being worked on now is post-game content.

As far as balance goes, I have an anecdote that comes to mind. Most people with a passing interest in nerd culture are aware that Pot of Greed allows you to draw 2 cards. Fewer are aware that the card is banned. It's a no-cost +1 to hand size and helps you find your key cards. Every deck would run as many copies as are allowed. Cards like Pot of Greed are usually an indicator that the people responsible did not know what they were doing when making the game. Anode Heart: Layer Null has a card that moves a unit from the graveyard to the deck and draws 2. The only cost is an action (you are limited to 2 actions per turn but have no limit on turns per phase) and the only requirement is that you have a creature in the graveyard. Its inclusion in the game suggested to me that the developers did not know what they were doing and that the game would have terrible balance. However, the game has a lot of depth, there are an astounding amount of viable decks, and no one deck is capable of consistently beating all others. I have experience breaking poorly made CCGs like the Pokemon TCG on Gameboy but haven't really found much to exploit here. The game probably isn't perfectly balanced but that hardly seems like a problem.

Anode Hear: Layer Null is a singleplayer CCG with a lot of depth and deck variety. It offers a ton of content and is very much worth the purchase for anyone interested in the genre.