The set up procedure is pretty easy and simple, following the on-site instruction, it took me a few minutes to get the program installed and set up from my PC. However there is a catch, your phone driver may not necessary be installed together with the package and hence you *may* need to install it manually by yourself afterward. It's a smooth and easy process, so no need to panic.
Once I had set up everything, I then follow the tutorial and implement my first Android App - "Hello Purr".
The App Inventor itself basically is a cloud platform that's driven by Java technology. The App Inventor has 2 major screen, the Design view and the Blocks view.
Design view is more like a WYSIWYG editor, you can see a dummy phone screen, user can place in (drag-and-drop) different components like images / sound / social widgets / buttons etc (from the left). You cannot implement / set any actions for any of these components just yet, instead user will be able to update the color and font size (for text). Under the Block view, that's where user can create actions and interactions for the components (that they had dragged in under Design view).
Once you are happy with your creation, user can use App Inventor to package their (own creation) app and store on the user's phone (via the 'Package to Phone' button on the top right of the Design view). App inventor made the Android app creation really simple and handy. Since I just got on the first tutorial, as a result I yet to explore the other components like social media widgets, sharing & Sensors like Acceleromenter and LocationSensor etc.
In short, using App Inventor to create Android App is more like using DreamWeaver to create your HTML page. You do not need to know in depth of how things work nor knowing Java language, yet you can still create a very decent Android app. Some people doubt about the ability of the App Inventor, my suggestion will be give it a try and see it yourself.