Check out this list of mostly obscure and unknown software that uses Qt.
Most software is obscure, full-stop. Just because you don't use most (or even any) of the packages on that page doesn't mean that Qt isn't a viable mainstream library, or that there's anything wrong with it.
Qt, like any other large framework, has a learning curve. If you're writing an application that works just fine using whatever libraries you're already using and you're only targeting one OS, then you probably aren't motivated to go climb that mountain. On the other hand if you're writing software (possibly with a complex UI) that is intended to target multiple operating systems, then Qt is probably the single best framework out there for doing so. Otherwise you're in for a long haul of writing your own less functional version of some subset of Qt features in order to abstract platform specific code away from the rest of your application functionality.