I had the good fortune to work on a code base that started as a blank emacs buffer and was compiled with both g++ and clang++ from the get go (and built on multiple platforms, with maximal warnings enabled). The two compilers surfaced different bugs in our codebase which, in addition to finding actual (semantic) bugs has been quite valuable for portability and maintainability.

(Also, as others have noted, the existence of clang has really been good for gcc).