[boost] The status of Proto

Hi, It is exciting to know Proto helps the development of expression template based library. Since learning it and use it efficiently could take quite a lot of time, I want to know whether it is almost stable right now. Or it will subject to some further changes in the future. Does it have any limit on what kinds of expression template based designs it can be used? If Proto is good enough, can it be used to reimplement many expression template based pacakges, boost.ublas, blitz++, etc.? Thanks, Peng