So I got my Illusion to 90 and I have Animage, Hypnotic Gaze, Kindred Mage, Aspect of Terror, Rage and Master of the Mind.
So my all powerful wonderful mind control works on everything I've come across as of level 20.

I am assuming my spells don't work on Dragons. Is there anything else these wonderful lockdowns don't work on?

Dragonpriests perhaps? But at level 90 Illusion, you're basically the Prince of Madness already. Rather that it works on almost everyone than that it wouldn't work on most of your enemies.

I thought Dragon Priests were considered undead. I haven't bumped into one of them yet either.

If you ever get to level 45+ (50+ is more prevalent), there are a whole lot of things that ignore illusions, unfortunately including most guards.

As a lvl 50 pure mage with 100 illusion and almost all of the illusion perks, it's pretty much useless on anything I'd want to use it on....
*summons two dremora lords"

Do you not dualcast?

Is the issue the level limit of the Illusion spells? I guess these spells only work up to 49?

Most enemies don't even go up to level 49.

That is sort of depressing.
Right now Illusion is completely amazing.
So past level 50 the mobs are immune or resist all the time?

Yet if you're level 65, every single enemy you meet ignores your illusions.
Most enemies with specific level ranges have higher level replacements.

Because you're using low level illusions or not having them properly perked and dual casting.
And this all stops about 50. There are no enemies designed for post-50.

Most Druagr stop at 30.
http://elderscrolls.wikia.com/wiki/Draugr
One class of Druagr hits 40. Another hits 45.

I thought dual casting only increased the duration.
But that might just be with Conjuration or Alteration.
So dual casting Illusion increases the chance to land the spell?

Dual casting is supposed to double the level it affects. I have no empirical numbers to back that up, but that's what the guide says.

Hmm, if Illusion is stable at higher levels it might be really fun to give it a go given what you said Xannon about its CC aspect.

I guess either way it will always have a use, either through CC or by making them Lords you summoned stronger.

At level 20 it is amazing. I'm Stealth and Illusion right now.
Cast Fury stealthed from range with Quiet Casting and watch them kill each other. Calm the last half dead mob, walk right up to him, crouch behind him and BOOM backstab. If the backstab doesn't kill him I just repeat until he's dead.
Full magicka, stamina and health and I only get my hands dirty for the fun of it.

I'd just hope I can keep doing this well into the later levels.

If you focus on it, it is too good, if you don't it does nothing. Not a fan of how it works, but without introducing a resist chance for all enemies like a D&D save system I don't see a solution. It really isn't better to just says all of these enemies are immune.

Oh and as far as I can tell dual casting does double the level it effects, or at least it increases it a lot. Before perking it I had about 10 free perks, with dual cast I was able to calm enemies I couldn't before, some enemies required both the other level boosting perks and dual casting. Some high level enemies may end up being immune because not enough level boosting perks apply so that doubled is not quite enough, like high end undead.(neither animage or kindred apply)

So you just found out why I stopped playing my Illusion-Assa. Its so frucking borring.

so illusion does work endgame if perked fully and dualcasted? If so then I am pleased if not then I am sad

It seems to. My tool tip would imply fury caps at level 40? when dual cast, but I think it does not factor in animage/kindred since some creatures fall out of that description. Some things are flat out immune to illusion, like I have not calmed a dragon, yet to try it on a dragon priest but they may be immune as well. Other than that it never seems to fail if you have all the perks and dual cast a spell.

I don't find this boring at all. I am having a blast sneaking around, mind controlling everything in my path and slitting throats.

My character is female Breton named Ghost. She has the whitest eyes, skin and hair and a blood red warpaint hand smeared across her mouth.
Almost as though someone she had just cut tried to cover her mouth to stop her from casting a spell but she can quiet cast...
And that's how they found her when she was fleeing into Skyrim.

I love the control over the game I have will Illusion. Not boring at all.