I don't think they were state of the art even back then, but they were widely available, well documented, and produced fast, robust lexers and parsers. They are

I don't think they were state of the art even back then, but they were widely available, well documented, and produced fast, robust lexers and parsers. They are still just the ticket if you need to quickly get a programming language implementation off the ground. There are some things that they don't do so well. It takes hard work to get good error messages out of Yacc, and anything that you might prefer to solve using feedback between the parser and the lexer (such as JavaScript's use of newline to terminate a statement, but only if it makes syntactic sense) is awkward to do because of Lex's lookahead — the token you want to suppress has already been produced by the time you know whether you want to suppress it. But its important not to get stuck worrying about minor issues like these when the critical task is to make something that works. You can always plan to throw away the Yacc-built parser and replace it with something better when you have time.