Most Illusion spells seem to have a level cap on what enemies they effect (e.g., Calm works up to level 9, Fury up to level 6). With perks, you can up those level caps by a bit here and there. Even at their highest level, though, it seems that these spells top out before your character's level cap (e.g., with both Kindred Mage & Rage perks, the highest level human you can affect is level 47).

What I'm unclear about is how much NPCs/enemies actually level with you. Will even perked-up Illusion spells be useless at higher levels as enemies level with you to some extent? I know they can't all level with you, but it's unclear to me which do. (I just used Frenzy on some bandits even though I'm level 19, and it worked just fine without perks; a minute later, inside a dungeon, an Apprentice Conjuror resisted the same spell.)

Thanks in advance for any tips on this. I'd like to use Illusion, but I just want to make sure it's not a waste of perks.

I rarely use illusion, and I mean rarely.

Cheers

As far as I know, they don't level up with you anymore per se. The leveled lists have NPCs with static levels (For example, Bandits are always level 1, Bandit Thugs are always level 9) and the different types and the amount of enemies generated is based on your level instead (For caves and whatnot, there are set level ranges. So a level 1-5 cave will only spawn enemies that are level 1-5, regardless of your level and a 30-35 cave will only spawn enemies level 30-35, regardless of your level). Some NPCs like Guards scale with you, but seem to stop at 50. Probably so you can't just Frenzy or Calm guards once you are level 50+

You can read more here:

http://www.uesp.net/wiki/Skyrim:Leveling#Effects_of_Leveling

Awesome, very helpful – thanks!

With the boosting perks (including Dual Casting) you'll be able to affect pretty much everyone/thing in the game, since you can push the 'X' in 'up to level X' to about 94 depending on what you're using and who/what you're targeting. You have to use the Master spells to get that high, but practically speaking the Expert ones will do; they can be pushed to about level 74 or so, and very few opponents are anywhere close to that level, let alone above it.