In my opinion s-exp have advantages respect XL syntax:
* precedence is explicit;
* you can nest domain-specific languages (DSL) because the first element of an s-exp can play the role of the DSL selector;
* it is easier creating multiple well behaving s-exp syntaxs, than for infix syntaxs, because in s-exp the syntax is weird in any case, so also the new DSL syntax is not less elegant than the host language :-)
* you can edit s-exp code using structural editing, like par edit, and it will work for any DSL syntax;
Maybe there is a middle ground between s-exp and XL approach. I know that Racket is experimenting with a parens-free alternative syntax, but I didn't studied it.