I'm currently using a second MBP as external monitor.
I've installed Air Display Host on my primary MBP and run AirDisplay.app on the second MBP. The AirDisplay.app is the client program.
The $20 license is supposed to be used on the secondary device, so I've entered it in the AirDisplay.app.
Both MacBooks are on the same wifi, I just told my primary to connect to my secondary. At first, it entered in a 'window mode' on the secondary, so I clicked full screen (using the mouse pad of the secondary ;)).
It did not behave as expected, as it started using my secondary as my primary: i.e., my dock moved to the other macbook. According to the FAQ of the developer, that is something they could not automagically correct, so I just changed this behavior (using the trackpad of my primary macbook) via System Preferences > Displays > Arrangement. Just drag the dock from one box to the other.
First thoughts of the performance: it's okay, but could be better. There's a bit of lag, clearly there's compression on the data stream, as I can see artifacts on my pdfs when scrolling. I wonder how to improve the performance. Don't get me wrong, it's good enough for writing/reading.