Do students simply work through hundreds of the Investigations one after another all semester long?

Certainly not. The investigations are not meant to be worksheets. They should not seem like an endless march of the students will get "IBL fatigue." The investigations are supposed to guide discoveries. In our classes we mix things up often - in terms of both mathematical content and pedagogy. Mathematical asides can be short modules from a different book, some breaking news from the world of mathematics, "cool things," a video, etc. In terms of pedagogy, there are some mini-lectures, larger project based tasks , poster sessions, etc.