I suspect that your working code does not exactly match the code that you posted above. Trying to guess some things, I encourage you to test your code against a JSON value that contains strings containing a single quote / apostrophe character, a literal backslash character, even a newline character.

Update: Oh, much later I realized how the code you posted could actually work. Valid JSON strings are also valid JavaScript source code. This might well open up a vector for doing cross-site JavaScript injection attacks, though that is likely true using your old 'eval' approach as well.