I am in the process of uploading a GCC 4.8 preview, using MinGW-w64 trunk. It is meant to be a preview of a release build, no experimental features were enabled for this build. The "unstable" means "soon to be very stable" and should be nearly finished/bug-free. Of course, I am optimistic and broader testing (by you!) will prove me wrong or right.
I'm hoping MinGW-w64 v3 will be out by the time GCC 4.8 is released, so I am not forced to withdraw to the v2 stable release, which does not include libstdc++ std::to_string and improvements in the C++ I/O.
If there are any issues, please let me (and the MinGW-w64 devs) know about it, so they can be fixed before the final release!