I've been using PMI's Organizational Project Management Maturity Model for my current client to establish best practices in the Portfolio, Program, and Project domains. They had previously done some process improvement on one large project using CMMI, and were disappointed in the results.
We've been very successful using the model, and it makes it quite easy to plan your process improvement initiative, and map those activities back to the Best Practices in the OPM3 model. In fact, we can measure increases in the organization's maturity "live" as process improvements are made. The way we have planned it, it isn't necessary to a complete RE-assessment after a period of time as long as we keep track of changes/improvements as they are made!
Has anyone had similar experience with OPM3, or using any other maturity model?