It is crazy good. It's permanent, (or at least it's lasted a very long time for me so far, so I assume it's not going anywhere), and it also applies to coins won in the slots. It also stacks with the double coins from the slots, because it's applied after the game, on the play again screen. I don't think it applies to the coins won from missions though.
Mantine loves all, gii. http://i39.tinypic.com/25kk0t3.jpg