Quickenings are ridiuclously game-breakingly powerful if used correctly. I was able to take out most of the story bosses purely with quickening chains once I had learned the vast majority of them.

The best way to use them effectively is to make sure you keep all 6 of the characters at around the same level as you play. When you get into a boss battle, unload a quickening chain in the usual manner while trying to get as many hits in as possible. Once it's over, trade out one character with a fresh one, and start another chain with that character. Due to the way the system operates, the other character still randomly receive mist charges even after their MP has been depleted, so you should be able to build up another chain of very similar length to that of a party with full MP.

Rinse and repeat for the other two reserved characters and you'll have set off 4 quickening chains. As long as you build them all to at least an average length (which, statistically speaking, you should) there isn't a whole lot that can stand against it (once again, in the story section of the game. Marks will mop the floor with you if you try this stratagy).

I found the volatility of Quickenings to be distasteful. They all lovely and flashy et cetera, but can't compensate for the individual lack of power. Yes, the player can rack up an awesome amount of damage to singular/multiple foes, but it's not always a dead cert that you'll inflict the desired damage at all.

It added a level of difficulty to the combat system, but from a general gaming point of view it sucked. Waiting for the ability to use powerful commands (like in VII for example) is by far more favourable for me.