Since this IDE is made by the Qt team for Qt development, you'd expect it to be well suited to the task, and it mostly is. However, I have two gripes.
First, the UI is quirky. The look and feel doesn't conform to any platform's standards, and it can be enormously confusing–even after you've been using it a while. Changing the background color helps the usability a bit (i'm currently using an olive green), but I do wish they had gone with a more conventional setup. Another problem with the UI for me is the way it switches to layout mode when you select a *.ui file for editing. When doing so, ostensibly to capture as much screen real estate as possible, the file browser gets hidden. When you are done with layout stuff, getting back to editing a code file is a multi-click affair: click on the Edit option of the main navigation, then double click on the file you want to edit. In other words, it doesn't return to the file you were most recently editing. Instead it shows you the XML of the *.ui file you've just finished diddling with along with a message that says you can't edit *.ui files in text mode. If I can't edit *.ui files in text mode then please go back to whatever it was I was working on before I switched to layout editing! (If I want to look at the XML, I can always do via the context menu on the *.ui file entry.)
Second, the equivalent of Qt Designer is built in, but I still prefer Qt Designer for design work because (1) screen real estate isn't eaten up by the IDE's other bits and baubles and (2) the form modeling in Qt Designer gives you a fake title bar that makes visualizing the final form easier. You can open any *.ui file in Qt Designer from within Qt Creator via the context menu (much as you would to hand-edit or view the XML in an external editor), but it's not necessarily the easiest thing to do. It would be nice if the Qt Creator had a “open ui files in Qt Designer by default” option. In addition to giving me a choice of layout tools, such an option would solve one of the UI quirks mentioned above as well.
Code completion is pretty dang good.
One thing to note is that this is actually a powerful IDE. The designers have done a decent job of exposing the most used stuff and getting the less used stuff out of your way. I think for this reason there are people who think this is some kind of IDE-lite.