I use xap in my home to allow for easy communications between the Homeseer server, The HT control system and 5 in wall touch screens that are actually windows 95 machines (Panasonic CF-01). I wrote all my own code based on the xap activeX control.
The same basic application runs on each platform, with variations in the message action routines.
For example xap messages run between the touchscreens and the homeseer server to control x10 devices, virtual devices, trigger events, run scripts, etc. Homeseer returns function resulkts via xap messages. If the Home Theater system (HT) needs to control X10 lighting it sends xap messages to homeseer. It also keeps Homeseer apprised of what is playing where so that anouncements can properly play over the dedicated anouncement system or through whole house audio if it is running, and in the theater if it is being used.
What I really like is that it is easy to trace messages to see what is happening.