As a casual Illumineer, my meta knowledge is foggy at best
Ramp You can build ramp decks in various ways, like relying on mana rocks or ramp spells
Illusionist's Gambit can absolutely save you while ending the night for someone else, but this is the poster child for a 4-drop that will rot in your hand while you wait for a better time to cast it
Convoking a huge seven mana spell with a horde of tokens that will then get to safely untap and swing into still-rebuilding boards
It's not a card you want to play on turn 3, but come turn 5-6, it has the ability to be a game changer, whether you're flashing back an important early game spell, or using the Harmonize ability to cast an expensive spell at a reduced rate
I think they're fantastic and have a lot of useful utility