Scilab

Pros: The level of compatibility between Scilab, Matlab and Octave is close to 99% which make transitioning from one to another a very easy task. The portability of your codes between these three packages is straightforward. The learning curve is pretty much flat.
Scilab is available on all 3 platforms (Windows, Linux, and Mac OS X)

Cons: Overall, Scilab's performance is less consistent than that of Matlab or Octave in most situations. Of course, it depends on what type of problem you're working on, but for my particular needs, I had always better performance (computational time, memory allocation,...) with Octave than with Scilable. Also, one of the limitation is that Scilab, has much less toolboxes and libraries than its two counterparts. If your looking at basic use of Scilab then you will be fine. But if you're an advanced user, you may feel the limitations.

Overall: I had used Scilab back when I was a student, It's an open-source numerical computational software with high-level math-oriented programming language very similar to Matlab and Octave. With only a few difference, the language is actually compatible between these three software packages. The level of compatibility between Scilab, Matlab and Octave is close to 99%. Overall, Scilab is fairly descent and does not stand out as compare to Octave. If you're looking for a free alternative to Matlab that's a fair alternative. But personal take home message: if you have the money (or the licenses) use Matlab (better performance, more features, more powerful). If you want something free use Octave (tiny bit better compatibility with Matlab, slight better performance than Scilab, and more fetaures and toolboxes than Scilab).