I have the same problem listed in the FAQs as being fixed.
It successfully compostited and exported the first time. Now it hangs when joining just two images. Sometimes it nearly finishes and shows the joined images, but then hangs.
I've uninstalled and reinstalled.
Oh, and this is on Windows 7.
This is a known issue in Windows 7. It will be fixed in the next Win7 release. I unfortunately don't have a schedule for when that is.