About “special purpose”, a great thing about Lisp is that you get the best of both worlds. Common Lisp is a general purpose language. But when you want/need a special purpose language, you can just build it in Common Lisp, using macros and other facilities. That’s hugely faster than writing a whole new language system from scratch! A lot has been written about these “little languages” and it’s one of the most important things about Common Lisp both in theory and in practice.

At my shop (ITA Software), we use C primarily when we have to do a whole lot of bit-level manipulation – I’m talking about huge amounts, where the performance benefit of C’s special-purpose ability to do this is overwhelming. We also have used C for writing Apache modules and stuff like that. But it’s the language of last resort. We use Java and Python a lot, too, though there are more lines of Common Lisp than anything else.