I stayed at the Mirage last month, and as previous posters have said, the front desk agent checking me in told me that the resort credit could be used at any MGM restaurant.
However, while the charges from the various Mirage restaurants were auto-removed, the Bellagio buffet charge was not. I didn't use up the full $150 credit until early morning of the checkout day, and those charges were not auto-removed (even though some were from the same restaurants that had been auto-removed earlier).
When checking out, I simply told the agent that the most recent restaurant charges were not credited, and she removed all the remaining charges, including the Bellagio buffet from the previous afternoon.
BTW, I believe that the credit works as a "comp" so the meal is free, and thus no tax is due on the free item. I had slightly >$150 in charges with tax, but since everything was removed, I think you actually get $150 worth of pre-tax credits. Or it could have been due to the manual application of the credit at checkout - the way to test this is to have only Mirage restaurant charges and see how much is auto-credited.