No more eclipse files scattered in your android repoThe plugin will create Eclipse projects in your Eclipse workspace and create links to the relevant content in the android repo. This means that you wont get eclipse project files (.project, .settings and such) spread out in your repo. The same goes for the build output. None of the compiled classes will end up in your repos out folder, they are for internal Eclipse use only.
Java projectAll the projects are set up as normal Java projects. You don't even need the official ADT plugin installed. You will be able to use all your normal Eclipse goodies like reference searching, code completion, pop up Java docs, on target debugging and what ever you are used to do in Eclipse.
Build AutomaticallyYou can have Build Automatically checked in Eclipse and see your code changes being compiled immediately. The first time the project is built it might take some time, but in the future it will only compile your changes.
What is it not?
Complete Android build environmentIt is not possible to build the Android deployables from within Eclipse. You will still have to build with your usual make scripts. (This will also mean that you will have to build with make when you change resources before you can see the changes in the Java source)
Native development environmentAt the moment you wont get any C/C++ projects, but you can still open any native source file with open resource [ctrl + shift + r]. You just wont get all the Eclipse ide goodies that you get with a proper C/C++ project.
How to get it
The source code is published on github, you can find the project here there you will also find a wiki and issue list. If there is any functionality that you want to see don't hesitate to create an issue for it.