If playing for winning or full collection, it's probably not worth to buy Illusionist. It seems to be a bit bellow the power curve.
The main strength of the illusionist+/++ is its power of a short rook, as that it plays as minor piece. Illusionist+++ plays very simmilar to a non rush rook and is a major piece.
Illusionist0 is most simmilar to minions like Drake and should play out simmilar.

Illusionists different unique mechanics are pretty awesome, but they aren't strong enough to be an important part of his power.

Anyway those make him able to do the following:
He can disrupt opponents formations, especially formations of immobile minions, by swapping them with himself, or with his Illusions. His Illusion can be blockers for a short time and allow allies to make threads though their body.
Illusionist or his Illusion, can help check mating, by swapping the enemy King, if getting close enough to him, this can be useful against armies, which use prince, if the prince can't follow the King.
The Illusions give the Illusionist the posibility to get back to every square, where he was in the last moves, which could trick opponents who try to chase him down.

As a short rook Illusionist won't be very bad in most armies, but probably for every army there would be better alternatives to him as well.