Our C++/QT desktop application for Mac, Windows and Linux needs an installer. I\'d rather we have a single installer for all three platforms. I do know it\'s a bit tricky, I
After fully integrating both Izpack and InstallBuilder into our builds (using Windows, OSX, and Ubuntu 14 build servers for testing purposes), I will say I believe InstallBuilder is well worth the money (and free for open source projects according to wojciechka).
Izpack is a bit slow, a bit large if you need to package a JVM, and has an amateurish user interface. Version 5 (release candidate 3) was also not generating uninstallers properly. That said, as long as you use a 4.x version and require a JVM anyway, it may be enough for your needs. The extension interface is not terribly easy to deal with, but is almost infinitely flexible. The Windows installers do not register with the Control Panel uninstaller list.
InstallBuilder has a great, fast UI in the produced installers and has a serviceable UI for creating installers. The XML is pretty easy to deal with, too. Downloads are about as small as possible. It also includes nice hooks for doing all sorts of custom stuff easily. The only slightly annoying thing I ran into was that the Windows server required that I manually add some configuration to set executable bits on the other systems' packages (other systems didn't require this configuration).
Note: I was using the three-platform version of standard InstallBuilder (not InstallBuilder for QT).