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QDebug is the best way to go, as it presents out-of-the-box integration with the rest of the framework, doesn't make you dependent on 3rd party code and covers pretty all of the logging needs.
I'm not using Qt, but for logging I'm using a modified version of Dr'Dobb's Logging in C++. The original code can be found here.
My modifications are specific to the Microsoft Windows platform (fopen doesn't allow file read sharing) and can be found here.