I always wanted to make a mage, but sadly skyrim's magic classes are underpowered, especially destruction. But i decided to give it a go anyway. So far, i tried a pure mage with alteration,restoration,conjuration and destruction.
But illusion, i never touched it and i want to ask for opinions about

-how useful is this class if i were to make a battlemage in heavy armor, or a destruction mage in cloth using sparks
-What other classes work best with illusion?

sad about mysticism though, could have made a great class in skyrim

Just sparks? You're planning to spend your entire playthrough, with a novice spell?

Anyway, back to the subject. Illusion is the par excellence crowd control magic school. You can pacify, fear, frenzy, move silently & become invisible. Casting frenzy while staying hidden, will make the opposition kill each other. All you need to do, is be creative and like all magic schools, study your craft

ah i see. Btw sparks as in, the entire tier of lightning magic. And maybe some fireballs from the fire tier. I do intend on keeping a sword as a backup but i will stay clear of xbows and bows. I plan to master destruction and alteration though.

But from your statement it sounds like illusion is best for thieves and assassins, like festus.

Why, aren't mages allowed to sneak, assess the tactical situation and pick a way to resolve it? You're not any sneakier than a destruction mage, placing a rune in a doorway, to make his enemies step on it. But that's another subject.

Illusion is a magic skill, as you use magical means to conceal your presence and misdirect, without relying that much on your sneaking skill.

But that's not it's only use. Pacify or Fear can also be used to thin the opposition out, bring the battle to your terms and ultimately -if you want- avoid combat altogether.

Illusion is amazing. My Khajiit illusionist/assassin used it to...
Breeze thru the TG quest.
FLATTEN Astrid. Now semi-regrets, since he would have owned as a DB assassin.
Devastate multiple Drager deathlords.
Get through the game killing 'only' about 70 people.
And rescue Roggvir. Didn't change anything, but proved it possible.

The only thing I personally dislike about Illusion is the amount of Perks it uses to remain effective at higher levels in the Game. Illusion needs to almost have every single Perk, but most other Magic Skills can get away with five or six Perks and be effective.
I do not use it on most of my Mages for RP, but it is fun to sit back and play with your opponents Minds. Whodunnit multiple times in a Dungeon is fun.
I agree with your decision to use Shock based Destruction, even if Fire is significantly less Magicka intensive, nothing in the Game has a resistance to shock.

I used to feel pretty much the same about Illusion, until I settled to a build, able to accommodate everything I really wanted to use in the game. But yes, electrical spells are the allrounders.

From time to time however, I still switch to fire when it comes to undead and fire/frost combinations whenever I just feel like it -opposition, terrified by fire, but unable to move because frost has them paralyzed

I have a hard time believing the last part. As soon as you fury or step onto the stage they cut off his head. I tried to fear everyone also but that just seemed to have the head cut off also... If you do not mind sharing your method?

Also, its a brilliant school. Not just good for thieves and assassins but also Vampires and those who wish to silently cast their craft without plunging balls deep into action. Deserves more respect than the healing school lol.

now you guys made illusions so much more interesting. I do like the 'whodunnit' effect in dungeons.

If you're building a pure mage, you should use all schools of magic, they are all useful especially in tandem. Like someone else said Frenzy, Calm, etc. are very useful.

While Illusion is an all-or-nothing perk tree, it is arguably the most powerful of all the schools of magic. Couple it with Conjuration and you will rarely take damage.