The most significant addition to the 2.5 release is a new compiler. The compiler is a statically typed, type inferencing compiler for a simple little general purpose functional language similar in style to Scheme but designed for low-level "systems" type tasks - particularly heavily numeric programming tasks such as signal processing and graphics processing. If you think of "C" with closures, tail recursion and alike you'll be on the right track. In particular the compiler has direct support for OpenGL programming, and for the real-time development of hot-swappable DSP code. Documentation is, as always, a little lite-on but a very brief user guide can be found here:
I have uploaded a short screencast demonstrating the compiler interfacing to the AudioUnit infrastructure to provide hot-swappable AudioUnit kernels. This is similar functionality to v2.0 of Impromptu but the new compiler provides some significant usability improvements.