Several questions have been raised during Helen's conference about Octave.
The goal of this wiki page is to provide all relevant answers and much more.
Victor Munoz opinion (Octave is really useful)
- Sparse matrix algorithms
octave-forge sparse matrix implementation page.
- Octave / Matlab compatibility
For users looking for a Simulink replacement that is compatible with Octave, a good tool is SciCraft (http://www.scicraft.org/) that is a Simulink like tool that uses Octave and R as the computational backend.