I consider Extension basically complete - quite a number of people are using it in projects. The main issue holding me back from submitting it for review by the Boost community is that I wanted to make the Reflection library part of Extension in a natural way, and I've been struggling at finding a balance between flexibility, performance and readability for the Reflection API. Also, of course, the Boost library review process is a bit painful - I'd need to block out at least 100 hours of my time to make sure the documentation, API, code, tests etc. were correct and easy to follow.
My previous post shouldn't be considered as much an obituary as an explanation of the slow progress lately. As I mentioned, I haven't been able to make much progress on the Reflection/Extension integration and new Extension features partially because it's difficult to design an API without having code that requires it.
If you find any features lacking in Extension, or aren't sure how to get the functionality you need, feel free to describe your use case to me - I can add the relevant functionality or documentation.
Also, always feel free to send patches. Extension is in the Boost sandbox: