I found a link regarding a comparison between MuPad and Maxima:
http://homepage.mac.com/peso1/MaximaMuPAD/MaxMuP-1.html
The link contains a series of test to mesure the quality of both systems.
There are several tests where Maxima returns incorrect results, so I'm
posting this with the hopes that someone will address these issues.
One example is the following problem:
(C21) assume(x >= y, y >= z, z >= x)$ is( x = z );
(C22)
(D22) FALSE
That is the output for maxima. It should be true