I know someone who works at Google, and their initial impression of the code base was that it's the highest quality large codebase (or largest high quality codebase?) they'd ever seen. Apparently all the qualifications and reviews really do help.
I saw parts of the codebase while I was there. It definitely has dark corners. But given it's size and the complexity of the problems it's solving it's wasn't that bad.
Of course at any given time while I was there the team was running several different refactor/fix/delete/cleanup projects. It's a continuous and well funded effort.
