There are hundreds of optional missions in the game, and each and every one of them are worth your time. You'll get lots of special items, gear and materia for completing them, not to mention Gil and "experience" that will go towards leveling up. However, you may notice that there are difficulty levels affixed to each mission. Keep in mind that these fluctuate depending on your level, so if a mission is listed as "very hard" and you don't want to undertake it at the present time, it will become "hard" and then "normal" (and so on) as the difficulty level adjusts to your ever-raising level. This ultimately makes every mission in the game manageable at one point or another.