Only use I've ever found for it is switching PS with a lower PS ship that is being targeted, so it can shoot first. Vader with Decoy passing his PS to ACD Echo so he can shoot before Most who would target him.

Decoy seems like it would be great when you're facing a Rebel Captive.

Someone somewhere suggested using Decoy in combination with Expose and Airen. Suppose for a moment, you had a Green with Expose and PTL, a Green with Decoy and PTL, and Airen. At the start of combat, you decoy swap Airen and the Green-D. Airen then shoots at PS3 (after everyone else has), and then gives Green-E a free action, which he uses to Expose and + PTL TL. Now the Green shoots 4 R1 TL+F dice! Furthermore, he is only exposed from PS0-2.

has anyone ever used decoy in a useful cost effective way?

I've toyed with Vader and Echo with Decoy in the mix. It opens up some interesting options. Echo can fire at PS that isn't normally possible (9 or even 11), or Echo can take a different E(P)T.

But it is an idea to go on the shelf until the advanced buff so Vader isn't a costly PS 9 Tie Fighter and a bit more of a threat. And you need to be a better pilot than me.

Vader, Echo w/Decoy and Soontir will be something I want to try once we have AT and the x1 title.

Decoy is 1 range?

See, thats the same range as Swarm Tactics, so you might as well put Swarm Tactics on Vader, and sadly leash Echo to Vader, which actually, totally defeats the purpose.

All you get out of this (for a huge mobility cost) is an offensive EPT on Echo who already has good offensive and can put offensive options like FCS or Recon Spec or Gunner instead.

Decoy is 1 range?

No, decoy is range 1-2. It's one of the advantages of decoy over SWarm tactics.