After 4 years of absence I'm finally coming back to Qt development again. I'm quite impressed by the development it has gone through, but also a bit confused by Qt Quick, which got just introduced in Qt 4.7
For me it seems like something to build a quick, appealing guy with some logic in Java Script. However, if I'm going to build a C++ application, is Qt Quick also the modern way of building a GUI for that or should I follow the classic path by using the .ui files?
The answer really depends on the type of application you're planning to build and I consider the description provided on the page you linked to very accurate:
"Qt Quick helps programmers and designers collaborate to build the fluid user interfaces that are becoming common in portable consumer devices, such as mobile phones, media players, set-top boxes and netbooks."
At the moment, Qt Quick is certainly not the most obvious choice if you're planning on building a classical desktop application, an area of development which the Qt library traditionally excels at, and in this case you're probably better off using what you call "the classic .ui approach"; at the same time, I think that's the exact reason why Qt Quick was introduced: to add a new tool to facilitate the development (or rapid prototyping) of applications not focused on the desktop.
Qt Quick should fill a much needed gap but whether that means it will become the way to build applications will largely be determined on the type of applications you intend to build.
Qt has provided .ui files for a long time. These files are easy to work with when you need to create applications based on standard widgets. Designer makes it easy to layout the widgets and do other basic operations.
Qt 4.2 introduce style sheets (qss) that made it possible to style existing widgets. These styles are convenient ways of styling the standard widgets to get away from the standard widgets' look-and-feel, while retaining basic functionality.
But sometimes you need to create custom widgets, widgets that don't exist. Before Qt quick, the only way to do this was to write native code (be it C++ or, with language bindings, Python or Ruby). With Qt Quick it's now quite possible to write widgets and full blown UIs that would otherwise be somewhat painful to write in code. Because Qt quick's focus is on the UI, it's becomes a better development language for that goal. The Qt Quick paradigm also transcends the idea of strict standard widgets, adding support for transitions and boundary-less widgets.