Earlier quoted context omitted.
I have. It used to use X11, I saw even more complaints then. There aren't any competitors, however, so people use Mathematica. I use high-end expensive that are Qt-only too, but I would gladly switch if there were other options, because Qt is buggy, uses non-intuitive non-standard UI, exhibits poor performance due to impedance mismatches between the Mac and Qt event/threading models, and ultimately decreases overall…
Can you give me an example of the "non-intuitive non-standard UI"? This may well be true, but it's quite possible it's just the developer was lazy and didn't do all the effort to make it look native. Edit: in fact, I'll give an example: VirtualBox - it does look crap on OS X, but that's because the developers obviously didn't make any effort to make it look native. They've got icons in tabs pane buttons and huge tool…
I've literally never been fooled by a Qt app -- a Mac user can spot that hot mess from a mile away. It's not just a matter of visual layout and UX, although that matters and always falls somewhere between subtly or completely wrong.
The controls also tend to lag at weird times, behave slightly strangely, demonstrate unusual font metrics, etc. The apps tend to exhibit bugs related to menu bar event handling, window management, on and on, and crash more than any other apps I use daily.
This isn't a huge surprise given that Cocoa is designed to be the first and only gatekeeper between events, the OS, UI controls, and Cocoa's main event loop, and most of what makes Mac apps a Mac app has to do with the APIs and integration that you lose first-class access to once you deploy Qt -- that includes 'simple' things like UI layout metrics, which Qt has to re-implement.
I abandoned almost all those Qt apps on my list above. I even dropped EAGLE, despite being "native", in favor of Altera on Windows, which I paid a hefty premium for in no small part because it wasn't a pile of broken UX, Windows or not.
I still use IDA Pro, but the minute http://www.hopperapp.com/ can replace my IDA Pro usage, I'll switch in a heartbeat.