Maras and other immunity amulets can be amazing in GRifts. Poison (Maras) and fire (Star of Az...) have been most useful to me, but if you dig you will find discussions about some of the others. Essentially if you run (fish) enough GRifts you *will* get a GRift full of monsters spamming (healing) you with poison and then get one of the poisonous Rift Guardians (which will you will pretty much be immune to). Same with fire and to a lesser extent some other types of damage (some folks would prefer lightning or arcane protection but just doesn't play out as well for me personally). Fire immunity used to be OP but Blizz has left several gaps in the fire immunity to make it less so... don't be surprised if you get killed with fire damage while wearing a Star. Even so, a lot of reflect damage and explosions that might kill you will heal you while you are wearing the fire immunity Star ammy.
You can also rotate through immunity amulets to run ubers or while running specific boss runs if they are giving you problems. Notice the type of damage they use as you are fighting them and wear the appropriate amulet (e.g. siegebreaker/Kulle uber is mainly fire damage).
+ Having said, if you are running a build that can use a hellfire amulet (almost any build not using Travellers/Compass Rose) then you will most likely want to use a hellfire to gain the additional passive skill.
So keep any decent immunity amulets you come across, but make it a priority to craft a good hellfire amulet (good attributes, good passive) if you can use one in your build... and don't forget that you can add an immunity amulet to the cube and equip it there. Good luck.