Is PyGTK still relevant?

The short answer is no.

PyGTK is being phased out and replaced with PyGObject as the widget set upgrades from GTK+2 to GTK+3. As of Oneiric all 'native' ubuntu applications run using the GTK+3 set.

On a personal note, fairly recently I started developing an application in Python, and having looked into it I decided to use Python3 with PySide (which is Qt4 instead of GTK+3) because I found that Python 3 would be the standard in the year or so it took me to write the program, and because Qt4 has a some very good documentation, and some first class development software (just have a trail of Qt 4 Designer...)