It's a complete jumble.
The missions thing is either incredibly tedious or incredibly wonderful depending on whether you like CC or not. It's little levels full of bad guys based on the same seven maps over and over again, with different start and end points.
ZOMG Spoiler! Click here to view!
Doing the missions completely overpowers Zack, and gives you ridiculously powerful materia.
But the availability of the missions depends on how far you've got through the game, and whether you talked to EVERYONE before leaving an area for the last time: there's no world map, you are where you are until you leave and then you never come back.
ZOMG Spoiler! Click here to view!
Talk to everybody in Midgar before you head out with Sephiroth. The betrayal and five year imprisonment by Shinra makes it kinda hard to return.
There's no indication ingame that not talking to everybody before you leave makes a specific mission slice not appear, but I can assure you that I'm right.
I can't get 100% now because I didn't talk to some pleb about clearing Midgar of monsters.
There's lots of materia.
ZOMG Spoiler! Click here to view!
Canny folks will figure out how to use Materia Fusion to make Zack super powerful with lots of super skills. Well six, because that's all he can carry.
I really like materia, so I liked Crisis Core.
Materia! It's magic, and it's a fun word to say.
Ask me about Materiaâ„¢.