A 5-mana goblin that copies a creature once a turn — and the moment that creature untaps things, or has an ETB worth repeating, that "once" stops being true.
Untap-and-copy combo shells. Restoration Angel or Deceiver Exarch beside Kiki-Jiki is the two-card version of an infinite army of hasty tokens — one of the most recognizable combos in the format.
One-shot ETB value. Even with no combo partner in hand, copying a Solemn Simulacrum, Eternal Witness or similar value creature once is a real card advantage swing.
Goblin and legendary-matters tribal. He's a legendary Goblin himself, so decks caring about either type get a body that also does something.
How it does in Commander
As the commander
Kiki-Jiki is rarely the commander — red alone doesn't support the enchantment and creature package a combo deck usually wants, so he's almost always found as a combo piece in someone else's 99 rather than leading his own deck.
In the 99
This is where he lives: a two-card combo finisher slotted into an Izzet, Naya or five-color list that already wants Restoration Angel or Deceiver Exarch for their own sakes, so the combo is a bonus on top of cards that are good anyway.
Formats it's strong in
Midrange and value-creature decks. The combo pieces are individually strong cards, so running them isn't a dead draw even when the loop doesn't come together.
cEDH and high-power pods. A two-card infinite combo that ends the game on the spot is exactly the kind of line those tables are built around answering or executing.
Formats it's weak in
Decks with no flicker or untap effects. Without a partner, Kiki-Jiki is just a 3/3 haste for five mana that copies a creature once — unremarkable on his own.
Low-power or precon-level pods. A hard two-card kill can be an unwelcome surprise at a table that hasn't agreed to see combos, which is exactly the Rule 0 conversation worth having first.
Synergistic cards
Restoration Angel — Flickers Kiki-Jiki when copied, which untaps him — the cleanest version of the infinite loop.
Deceiver Exarch — Untaps target creature on ETB; targeting Kiki-Jiki with his own copy closes the loop the same way.
Zealous Conscripts — A copy that also steals a creature is a real answer to a problem permanent, not just a combo enabler.
Felidar Guardian — Another flicker-on-ETB body that reads the same way Restoration Angel does with Kiki-Jiki.
Is it worth the price?
Kiki-Jiki has had enough reprints to stay affordable for what it does — a real infinite-combo piece rarely stays this cheap. Worth owning if you're building anything that already wants Restoration Angel or Deceiver Exarch; not worth chasing for a deck with no way to untap him.