Trance is not even useful like Limits or Overdrives. That's what annoys me. I'd be perfectly fine if only the Trances were better. Way better.

PS: I don't care much about Attack boost that every character gets. Seriously.

Zidane:
Dyne is kind of stupid. It's only physical damage and nothing else. Okay, it's GOOD physical damage, but even so... plus, Dyne kicks Skill, and later on Thievery can deal more damage than EVERY Dyne by LESS MP than all of them.

Vivi:
Double Black helps most of the game. But it's not OMFG LIFESAVER!!!111oneone, unless you Double Drain/Osmose on something. Well, Double Stop is also neat on a Lv1... @_@

Steiner:
Attack is even more boosted than the other characters when on Trance. However, unlike Dyne, it doesn't kick out your great end-game skills. But it still gets obsolete... quickly.

Dagger:
Free additional Summons, more chance of full animation if no Boost. I like it, but it is no life-saver either.

Freya:
USELESS. Her Jump causes random-ish damage, and she won't come down until Trance ends. And she could be using Potions and Hi-Potions with that neat Chemist skill.... x.x

Quina:
Cook is NOT necessary. Eat is good enough. At least, I can't think of a moment where I just can't reduce the monster's HP to 1/8 without killing them.

Eiko:
IMO the only really useful, life-saver, really nice Trance. Double White is awesome. Curaga + Haste. Double Might on someone. Double Holy. Two (Full-)Lifes. Awesome.

Amarant:
Multitarget Chakra and Aura would be awesome... if Amarant weren't so SLOW to get on Trance. I only see he Trancing 3~4 times per game. x.x

and btw, Stealing on FFIX is only useful on Bosses. That also annoys me a little. But at least Steal WORKS... unlike older FFs like IV or VI. *shudders*