Ajani, Caller of the Pride is a pretty good planeswalker for only 3 mana (1WW.) This Ajani planeswalker fits well in just about any white weenie or soldier based deck.
Ajani, Caller of the Pride's +1 loyalty ability is certainly not spectacular, but those +1/+1 counters do add up. He starts with 4 loyalty right off anyhow. So for a plus loyalty ability, it's acceptable.
The -3 loyalty ability is probably the best one on this Ajani. You can use it immediately without losing him and give something flying and double strike. Getting in for a quick few damage is always a good thing.
The ultimate loyalty ability is a -8, which means you'll likely rarely use it. But if you do get it off, the 2/2 Cat tokens could become quite relevant. It's all about keeping your life total high enough to make it worth using.
He was very playable in Limited and was played in aggressive mono-white, green/white, and Naya (green/white/red) decks. Since he's only 3 mana to cast, you could easily play 3 or 4 of Ajani, Caller of the Pride in a deck easily. Most decks only played 2 copies, however, and mostly for that -3 abiliy.
This Ajani was always a target for removal, as he can pump one of your guys every turn he's out there. He'll always be worth the 3 mana, even if he's removed right away, because that's one less removal card you have to worry about for an easily replaceable card. He could also take some combat damage for you. Not to mention, as long as Sun Titan was in Standard, Ajani, Caller of the Pride would come back and say hello at least once in a game. He still pops up in the Modern format on occasion, as well.
The Commander applications are quite obvious. He's extremely strong in the 1V1 Commander format where his -3 can be absolutely game-breaking. Unsurprisingly, Ajani, Caller of the Pride has found himself in many Atraxa, Praetor's Voice decks, primarily for his Cat token creation ability. Also unsurprisingly, Kemba, Kha Regent Cat decks are happy to have him. But pretty much any aggressive deck is happy to have a copy of this Ajani planeswalker.
Is Ajani, Caller of the Pride incredibly game-breaking? Not really. But, he can certainly help give your army an extra push, and sometimes, that's all you need.