In my opinion, what makes them really great is the tight integration that they have. For example, since the whole company uses one build system and one single repository, you can build a truly awesome IDE that knows about every library in the company and can autocomplete for it. Same for code search, where cross references are accurate and work cross languages (for example a class generated from Protobuf).
Outside Google, the percentage of my coding time I spend hunting some dependencies source tree for the relevant header files or documentation or "Where on earth is this constant defined" is huge.
With codesearch, answering those kind of questions is near instant.
