The Blender 2.57 release also carries thousands of bug-fixes since the earlier Blender 2.5 beta releases. Key features of Blender 2.57 include it being feature complete, the new user-interface, and many other Blender 2.5 changes. There's also major improvements in stability. There's also much-improved Mac OS X support, including 64-bit support under Mac. Still missing though is multi-window support, a full RNA data level dependency graph, and radial menus.
Blender 2.58 is expected to be out in about two months time and will be the last of the 2.5 series. Following that, it's onto Blender 2.6x.
More information on all of the changes in this open-source graphics/animation tool release, visit Blender.org.