A big problem with transitioning to Python 3 is that there are quite a few third-party libraries that don't support it (including two I use regularly - SciPy and Pygame). Some bits of the syntax are different, but that shouldn't be a huge issue except for big codebases, since there's a script to convert Python 2.6 to 3.0.

I've used Python 3 but had to switch back to 2.6 so I could keep using those libraries :P