In our experience, scala-json turned out way too slow, as it allocates a lot. If you have lots of data to process, Xstream works well (
http://xstream.codehaus.org/json-tutorial.html), and
Jackson Fast JSON parser (http://jackson.codehaus.org/) is faster still.
Also, it appears that while scala-json adheres to the Stairway book, scala.util.parsing.json does not, and overall seems a premature choice for the JSON parser in the standard library.
Cheers,
Alexy