It's not a thing to take lightly when you take a program weighting several KLOCs you've been developing in a Microsoft environment (Win2k, VC++ 7), compile it on Linux and see it run in exactly the same way as on Windows, without a single error/warning/difference (bar the native widgets, that make the GUI look "at home" on any system).
Qt's famed cross-platformness works very well. So far we tried the application on Windows (2K and XP), Linux RH7 RH9, Solaris and AIX. The only problems we had were caused by our bugs and not Qt's lack of compatibility.
This is very impressive. And useful !
Eli Bendersky's website