Possibly the worst build system ever
Those who do not study make are condemned to reinvent it, badly.
Maven is an unusable, obscure, opaque and intractable build system whose strict adherence to some theoretical religion of "the ideal build system" is only matched by its total lack of debugging information.
Rather than providing a simple, configurable specification language for builds, it forces every single trivial task to be implemented as a Java plugin, and wraps those plugins in a syntax and invocation semantics so torturous and oppressive as to reduce any sane person to writing a shell script.
It fails to satisfy so many fundamental requirements, such as obsolete class dependency analysis, producing human readable test output, and so forth that one has to believe that the maven developers have never actually written any real software.
Maven is very good for one thing: Writing trivial maven plugins. If you want to build software, use make, or at worst, ant.
An open, cross-platform journaling program.
A scientific plotting package.