[MorinkhanMT]

In Survivor: Guatemala, Gary found the one hidden in the tree and played it.
In Survivor: Exile Island, Terry found it but never played it.
In Survivor: Cook Islands, Yul found it but never played it.
In Survivor: Fiji, Alex played it (but Edgardo was booted so it was re-hidden). Yau-Man played it and saved himself eliminating Stacy. I think Earl played his at final 5 since there was no reason not to.
In Survivor: China, James kept both of his and was booted, which removed them from the game as a result.
In Survivor: Micronesia, Ozzy was in possession of it when booted, didn't play it, I assume it's out of the game for good if the rules from previous seasons have stayed the same.

So, only Yau-Man has ever used a HII effectively to save himself from elimination.

Of course, Jaime in China and Eliza in Micronesia played "fake" idols. Great hilarity ensued. Jeff throwing Jaime's fake (the one Yau made) into the fire after she played it at Tribal remains one of my all-time favorite Survivor moments. =)