We are now officially in phase 1 of the release process for 1.6. this means the features are confirmed for release, as detailed here, and there is a 3-4 week period during which these features will be complete (functionally).

After that the codebase is locked down to implementation details, i.e. the externally visible "shape" of Aqsis 1.6 will be defined at that point and won't change.

Already there have been significant performance improvements during the 1.5 development cycle, as can be seen on the beaker results page, and 1.6 is primarily a performance update, however there are some nice functional changes, such as an overhauled piqsl interface.