As I put front and center in the Buck documentation: "Buck is a build system for Android that encourages the creation of small, reusable modules consisting of code and resources."
By design, Buck encourages you to create small modules so that you can easily compose a new app out of your existing building blocks. This means that maintaining multiple deliverables is straightforward: it eliminates boilerplate without requiring you to organize your repository into a predefined structure. You can also create ad-hoc build steps to suit your needs with Buck via macros and genrules. (A more formalized extension system is in the works.)
We also care a lot about speed, particularly the speed of incremental builds. Because Buck has a strong concept of dependencies, we can often avoid rebuilding intermediate artifacts. Other build systems also try to do this (like Ant), but frequently sacrifice correctness, as a result. We don't.
We recognize that IDE support is important. Certainly Google's collaboration with Gradle gives them a leg up there. However, Buck has a command to generate an IntelliJ project from the dependency graph defined in Buck build files, and we have broken ground on our own IntelliJ plugin, so this is something that we also care deeply about.
Finally, bear in mind that Buck is used to build Facebook, Facebook Messenger, and Instagram for Android. Buck is not going away. Further, the code for all three apps (and reduced versions of the apps, for even faster development cycles) lives in one Git repository at Facebook, so those of us working on Buck internally are sensitive to the needs of large codebases that support multiple deliverables.
