Is an Illusion mage a viable character choice? The reason I ask is... because Illusion is governed by Personality. Personality! So Personality is what... Speech, Mercantile, and Illusion right? By leveling Illusion I will be raising Personality, which I don't really care for because of that Speech mini-game and the fact that bribe does not count to level Speech, which is insanity!

In fact, I'm not sure Personality is all that useful. Really, I just want to calm people or make them attack their slobbering goon buddies while I take their things, or make them hungry to look for food or want to read. Can Illusion be used against undead? I don't think it would! Maybe look for an Illusion improvement mod. So, I'm asking about Illusion because I've never played an Illusion character.

Illusion is probably the most powerful magic skill in the game. Illusion definitely works on all forms of NPCs, Creatures, and Undead, although there are different spells that work on different enemy types (Demoralize doesn't work on Undead, for example, and Frenzy only works on Humanoids.) Some Illusion effects, like Chameleon, Paralyze, and Command, are so powerful that they can potentially "break" the game (removing almost all the challenge.)

You don't need to raise Personality if you don't want to.

"Governed by" is a deceptive phrase in Oblivion. The Attribute doesn't necessarily affect the strength of the skill it's associated with. You can maximize Illusion without ever putting a single point into Personality.

The advantage of high Personality is that NPC Disposition is better; everybody likes your character a lot more.