I think the instrumenting and generator stuff gets disproportionate attention. For me by far the biggest win from spec has been with parsing. This completely changes how you'd write a library that takes a data structure and parses it into something meaningful (for example, what hiccup does for html or what honeysql does for sql).
In the past, this required a lot of very ugly parsing code and manual error-checking. With spec, you write specs and call s/conform. If it failed, you get a nice error, especially if you pair it with expound. If it succeeded, you get a destructured value that is really easy to pull data out of. I've done this in a half dozen different libraries and i'm pretty sure i wouldn't have even written them without spec.
In the past, this required a lot of very ugly parsing code and manual error-checking. With spec, you write specs and call s/conform. If it failed, you get a nice error, especially if you pair it with expound. If it succeeded, you get a destructured value that is really easy to pull data out of. I've done this in a half dozen different libraries and i'm pretty sure i wouldn't have even written them without spec.