Your opinion on lex and yacc

[I've got 50nS memory. What did you say?]

I just read a letter about a course given by McKeenan at Wang institute. He
allegedly stated that lex and yacc are not good enough for "commercial quality"
compilers. I was always under the impression that they were high quality tools.
So what are your experiences with lex and yacc? Particularly as regards
implementing a C compiler. As it is not exactly a pure C question, please
mail me your responses to avoid cluttering the net.

dov