So we all know destruction magic doesn't scale that well into the late game, what about illusion? It seems like it would be totally useless after a certain level since they only effect certain levels and below.

If those "levels" are anything like they were in Oblivion, then they don't actually reflect the level that you're playing at or even your enemies' levels. It was an unintuitive system, but basically a level 25 illusion spell would affect any enemy at any level.

Coupled with the perks in Skyrim, I imagine illusion will be just fine at high levels.

Playing on Master as a pure mage. I don't get the Destruction complaints. My damage is excellent when combining Conjuration pets with Destruction single target. The Impact talent alone makes every point well spent.

To answer your question, I have alteration and illusion both at 100. They are both very lackluster. The alteration shields are simply too weak compared to armour, while illusion is more of a fun school where you can force enemies to fight eachother and turn invisible. Surprisingly not that useful for level-appropriate content.

Basically each of the above only has one really useful spell. Paralysis for Alteration and Invisibility for Illusion. It seems really wasted to level a whole school to 100 just for 1 spell, plus talent investments. However, both are very easy to level which offsets this somewhat. Illusion can be quickly brought to 100 by spamming Muffle, while Alteration can be quickly leveled by standing in the middle of a town and spamming Detect Life.

Long story short, stick with destruction. Level 41 on Master and it provides excellent SUPPORT damage. Just don't neglect Conjuration.

I am pretty sure your level of illusion has no effect on the spell. Spells have set levels they work on, and then there are perks that make those spells more effective. The way the spell system works in this game it seems after a certain level illusion spells will have no effect except on undead.

only conjuration magic scales and thats only to lv50.

all other magic type do not scale