Hi ! Qt is one of the C++ framework that I am using the most on the GUI since several years. Why so ? Because the way it is designed and built enable a very good productivity, I do not have too much time to invest in the GUI. Qt is well documented, maintained and hassle free.

Productivity

I do not want to spend too much time creating dialogs, adding labels, buttons and other stuff. In my daily practice I use almost exclusively the GUI Builder tool: Qt Designer. So you can have more time on more important or critical tasks. Link: http://doc.qt.io/qt-5/qtdesigner-manual.html

Handling internationalization is also very easy through Qt Linguist, you can give it to a translator directly saving developer time. Link: http://doc.qt.io/qt-5/qtlinguist-index.html

I am not a fan of IDE, I like to manage nuts and bolts of my devel workflow, but if you like this, Qt Creator is nice. Link : http://doc.qt.io/qtcreator/index.html

Documentation

I think it is one of the most important factor to decide to use a library. Documentation, code samples, tutorials are really key to ease the adoption and speed up all the learning process and practices. Qt is having probably one of the best documentation amongst any language, any library. You can have a look here to see here the big picture: http://doc.qt.io/. There is also a huge community of developers able to help you when needed.

Design

One of the key element to understand the success of Qt are the concept of slot and signal. When you build for instance a dialog box, you have few buttons, when for instance you click on one of them, a signal is sent and a part of your application just need to subscribe to this specific signal. The receiving part is called a slot. Qt enforces you to use such signal and slot abstractions to build cleanly GUI. Such signals and slots are new types attached to your C++ class and you have the moc tool that is performing the error prone and heavy burden of linking everything together and generating the glue code needed. Links :http://doc.qt.io/qt-5/signalsandslots.html and http://doc.qt.io/qt-5/moc.html

Libraries

Qt itself is decomposed in modules (libraries): Core, Widget, Multimedia, Network, Sql, Quick ... and it has a long set of libraries: Serial, WebSocket, Sensors, OpenGL ... Link: http://doc.qt.io/qt-5/qtmodules.html

Licensing

There is either an opensource licensing scheme ((L)GPL depends on the module or library used) or a commercial one. For the commercial one, there is special prices if you are a startup (less than 100K$), otherwise there are either yearly fees or perpetual licensing scheme too.