We just got finished with a few intranet-related projects, so this is really top of mind. Jakob Nielsen has some handy intranet usability reports online for purchase.His group measured usability in terms of the time it took to complete a few very common intranet-specific tasks, like logging in, finding someone in the directory, searching for a file, changing personal information, etc.
It's got some case studies, and 111 design guidelines for intranet design (whew!).
He is even so bold as to try to measure productivity and ROI on intranet usability (yes, you too can save $30.4 million by tweaking your intranet's interface!). I always take Nielsen's numbers with a grain of salt, but really do enjoy the usability testing he does.
Intranet Usability Reports from Jakob Nielsen...
Intranet Usability Reports
