I don't mind fighting games on the iPad, Infinity Blade proved to me that it's doable and you can have complex, fun fighting systems even with the limited control options. I wondered what kind of game Injustice would be on the iPad, being sure that it couldn't be a port of the hit console mortal kombat-like fighter. I was mostly right, it's not.
The fighting system is shallow and gets repetitive after the first fight. You basically have four moves and they always play out the same way. You can either tap the screen to do basic attacks, swipe left/right to do heavy attacks, hold two fingers to block or activate your super move when a meter is full. There is no strategy involved, you tap and swipe and use your special moves when you can and the enemy switches out to another fighter when they're close to dying and usually they unleash a special move right then, blocking should be more effective, you can't block while you're being attacked and all attacks are combo strings, so either you block while nothing is happening, or you can't block at all.
Swiping up and down should be used for something, you can add a few more moves that way, I'm sure these characters had plenty of moves in the console fighting game, more variety would've been welcomed.
Some characters have special abilities but they don't matter much in the long run. Catwoman deals extra damage to enemies suffering from bleed effects (she should've been able to make enemies bleed with her normal attacks or something) and Solomon Grundy can revive when he's down (with low health, low enough to be knocked out by a combo), most of the character's special abilities feel like they don't exist at all and besides basic statistical differences, they're mostly the same. You can equip various items to your characters to make them better in some ways, but these items cost power credits and you only gain a few per battle.
Speaking of power credits, I know it's something weird to argue about, but why not add a credit doubler or something similar? I'm never going to buy 100$ worth of credits (and that's not even saying it's enough to unlock everything in the game) but 1.99$ or something to get double credits after each fight would be very tempting because you gather them quite slowly and the things you can buy cost up to 100 000 credits. And then you have to buy support cards and upgrade your powers and all that stuff I won't bother with because I can still win fights.
Otherwise this is a fun little game for a while, even if the energy system makes you hit a wall after you go up a few ranks. Each fight takes energy out of your characters and you need to wait/pay to make the energy come back, the more you go, the more energy you need to spend to fight, so after a while you'll NEED to wait/pay, and that's not very fun, waiting while doing nothing isn't great gameplay.