I found GTK with Python really nice. With a bit of effort you were able to make it look native on all platforms. Clean API, visual Editor, and quite performant. Unfortunately the project suffers a bit from CADT (Google it) and the devs break backwards compatibly all the time. They seem no longer interested in providing compatibility across versions or for platforms other than GNOME unfortunately. With GTK 2, you could write an app with e.g. GTK 2.16 and it would still run with 2.32, but receive all new features from the toolkit. Now you are expected to constantly change your app with every release.