Inspired by the maturity problems I am facing porting on Mono Mac & Linux. I am investigating the use of Qt as an alternative. I am curious to hear about your favorite Qt experiences, tips or lesser known but useful features you know of.
Please, include only one experience per answer.
I've used Qt 4.5 & 4.6 for some applications such as TCP/IP game with a uCsimm and a shooting game with graphics. Qt made my life easy as I need to write code once and have it running on Windows, Linux & Mac.
A free & quality book to start with: http://cartan.cas.suffolk.edu/oopdocbook/opensource/ .
Qt may not have all the things that Mono/.Net have but for sure, Qt is much more mature than Mono and is truly cross-platform. FYI, Skype and Google Earth use Qt. I like how easy to create cool GUI, state machines & database-driven applications with Qt. Oh, yeah, Qt has great WebKit & Multimedia modules that get you on the fast track of web & media integration.
Give Qt a try and experience it yourself. It has awesome demos & examples, check them out!