It has been used professionally for over a year. Qt dramatically cuts down on development time and complexity in writing user interface software for the X Window System. It allows the programmer to focus directly on the programming task, and not mess around with low-level X11 code. Qt is fully object-oriented.
Reusable components can work together without any knowledge of each other, and in a type-safe way.
Qt has a very fast paint engine, in some cases ten times faster than most other toolkits. You have full access to low-level painting functionality. Painting is device independent, so the same code that draws on the screen can generate printer output. You can also do arbitrary clipping, rotation, and scaling - simply and fast. Qt is very fast and compact because it is based directly on Xlib and uses neither Motif nor X Intrinsics.