Double Agent is basically a cheap version of cloak where enemies can't see you nor attack you. As said above enemy minions, Auto Titans and even enemy Heavy Turrets will not fire upon you instead they view the player as a Friendly.
This card is EXTREMELY good in Frontier Defense as every enemy will ignore you. With this card in effect, you are practically invulnerable for the game unless you die by suicide, Arc Titan's chain/AOE electric field, Nuke Titan explosion, and Suicide Spectre explosion.
Double Agent may be effective for sneaking by minions, auto titans and as well as avoid being shot at by heavy turrets but remember this card doesn't fool human players so don't think you can just waltz on in enemy territory without a good fight, because you'll get a fight.