IUP is not a huge system like Qt/GTK+/wxWidgets and the code for GUIs is relatively concise. I think this lends to a perception that IUP is just a simple system

IUP is not a huge system like Qt/GTK+/wxWidgets and the code for GUIs is relatively concise. I think this lends to a perception that IUP is just a simple system for simple applications. Actually, it is focused on engineering applications, with some powerful widgets for data entry and visualization. The development team is small but very active and responsive. Windows is well supported along with various flavors of Unix. IUP doesn't seem to get much use on Linux. I think IUP doesn't insulate you from the platform as much as Qt does, so cross-platform work takes a bit more care. Not all the features available on Windows will be supported on GTK/Motif. Even if the only target is Windows, IUP may be worth considering for the clean code, convenient layout management, and the data-oriented widgets.