Not for data entry or creating JSON from scratch.

Cons:
-Frequent crashes
-Drops data
-Namespaces get dropped/renamed
-File saving seems to be partially broken

Pros:
-Looks, nice?

I can see how this might be good for opening and reviewing data in JSON, but for creating and modifying data it is useless.

The App Store really needs a "gimme my money back" button.

DrPunchman about Cocoa JSON Editor