dansul

Yes, to repeat what others have said... They are great against heavy cruisers, because you can keep them away. You do have to micro with the ability. You may not use it in every battle, but the chances are you'll have guardians anyway and Repulsion can be a lifesaver. Personally, I tend to use Illums and Guardians and when I hit t5, get Repulse and Deceptive illusions before upgrading to Heavy Cruisers.

Ilums are pretty good. They have turrets on three sides, so you need to position them in the middle of an enemy fleet to maximize their usefulness. That's where Repulsion comes in! The guardians can push the enemy fleet all around and keep them away from the illums.

Illuminators are long range too. Granted, not as long range as strikecraft, but the final upgrade for their beam weapons is a range increase.