Some (potential) context to this is that at Clojure Conj there was a discussion of the potential for clojure expressions to become a standard data serialization format. The idea is that clojure data is significantly more rich than something like JSON.
I didn't say mature I said they were a good format for it. And by what metric are you measuring maturity?
* Age? s-expressions predate python by a fair amount.
* Parsers? just about every language I know has a lisp
interpreter embedded in it. s-expressions therefore
parsers in pretty much all languages.
* Market Share? well xml and json may win there but I'm
not sure that's a valid reason to discount