I've used Numpy, Matplotlib, Octave and Scilab. None of them are really anywhere as good as Matlab. R has quite a different focus.

The biggest thing Matlab has is the GUI (which is surprisingly good!), the documentation (excellent), and the plot viewer. The plot viewer is probably its killer feature. There's just nothing remotely as good in any other software, at least as far as I could find.

Octave's is terrible. Barely any features and incredibly slow.