First, the Illusionist doesn't need to do Illusion Magic. It sounds silly but it's true and a lot of people make this mistake.
So, the Illusionist is only a "able in secondary habilities/wizard" class.
You'll be a little less efficient in Magic than a pure Wizard. But you'll get bonuses in some skills.
You'll earn less zeon per level, and you'll have a MA multiple more expensive.
I'll advice you to choose well your advantages/disavantages to fit your character concept, keeping in mind that you won't be as good a wizard as a pure wizard.
It's an "hybrid", so, depending of your character concept, you'll need to focus a little on one of its sides (magic or competences), or, you'll need to keep a balance between the two sides. I can't say more without knowing more about your character.