Any project needs a build environment. For a project like Impala, which is trying to provide a simpler environment for Spring development, the choice of build tool is important.
ANT has been around for an awful long time, but it's not exactly the most sexy software around these days. For a while it was the only show in town. Nobody likes writing build scripts, and the one thing that ANT does make you do is write build scripts for your projects.
These days, the obvious other choice is Maven. Maven's two selling points are thus: it saves you from having to write build scripts, and it is supposed to handle dependency management for you. Of course, it cannot do everything. You need to tell Maven what libraries you want to use. Maven in turn will find a whole bunch of other libraries that you don't necessarily know about, which the libraries that you do want to use themselves depend on. This is called transitive dependency management. The problem, though, is twofold. Maven relies on dependency information put in POM XML files. This information is only as good as the person editing the POM. It's not foolproof. Secondly, even if the POM is accurate, it may be more complete than it needs to be. How does it deal with optional dependencies? If these are included in the POM, but not needed for the parts of the library that you are using, then you get a bloated repository. And the one thing I can't stand is bloat.
There are other things I don't like about Maven. Frankly, it tries to do too much, without necessarily doing anything very well. Lots of people I speak to have complained about this. It entails a lack of control over your build environment, which scares me. The horror stories you hear about don't seem to happen with ANT. It's a pain, but one way or another, you know you are going to be able to get the job done.
A second option is Gant, based on Groovy. Gant is built on ANT too, but uses a Groovy DSL to make ANT scripting easier. It seems much better for complex scripting, because you can use proper language features such as iteration and conditionals much more easily than with ANT. Two problems with Gant, though. There's more setup involved, because it requires Groovy and Gant to be installed (as well as ANT, perhaps). Also, it's a bit slow. Groovy is much slower to compile than Java, which has a noticeable impact on build times.
For these reasons, I'm back to ANT. If I need to do anything complex, like handle dependency management, or iterate through projects, I'll write a custom task for that. It's easy enough. ANT is not pretty, but it is effective.