I just finished a four-week contract that involved development with Qt4. After developing in C with GTK+ for the last couple of months, this was quite a refreshing experience.
This was my first non-trivial project with Qt4 and I'm quite impressed how well it went. Compared to Qt3 (and even Qt2, which I was stuck with on embedded platforms for a long time), Qt4 is a major leap forwards, especially with regards to concurrent programming (Threads) and separating data management and representation (Interview). Note that you want to use at least version 4.2 though, because earlier versions still have a lot of bugs.
In my opinion, Qt is definitely the most-advanced open source GUI toolkit around -- it's probably also even better than most proprietary ones, although I can't say this with certainty, since I don't know anything about Cocoa... yet!