One of my favorite uses is to have it running MOG or other music apps while I am using the main screen for other things. I simply installed the app on the iPad, then installed an app on my MacBook Pro and I was in business. There are a couple setting to choose from. If you use the "Retina" setting for the new iPad, you will probably find it difficult to read. You don't have to restart the computer to change this setting, so I found myself turning it on and off depending on how I was using it.
While Airfoil serves a similar purpose (it will send an audio stream from the iPad to the MBP), I enjoyed the ability to just move the curser over to the iPad and use the MBP's keyboard and mouse to put in new songs or artists on my iPad.
Also, it is great for Photoshop CS6. I can put the Bridge browser on the iPad and use it to select photos. It will then open them up on my main display in CS6.
Is it worth $9.99? It really depends on whether you will use it. I really enjoyed having the extra display while using my laptop, so I think it was worth the money.