I try to use 4 kinds of metrics to measure success:
1. Activity metrics: whatever I can get out of the tools -- number of uploads/downloads, posts to discussions, etc.
2. Success stories and anecdotes: someone who was able to land a big customer, solve a problem, etc. thanks to our content management and collaboration tools. If you are looking to justify the system prior to implementation, you might turn it around and also look for stories of failure where the organization lost money or time because there was not a system in place.
3. Process improvement: this is where you can get unambiguous ROI if you capture process baseline data prior to implementation and then measure later to look for improvements in speed, quality, etc. Any processes that depend on having the right information at the right time are good candidates.
4. Employee surveys: asking questions along the lines of "how satisfied are you with the information you have to do your job" can be an indicator of the need for a KM system, and used as a baseline to refer back to after implementation. Periodically reissue the survey to see how or if things have changed. I also have occasionally done SNA-type surveys to see how connections between employees are changing.