I've been using Python 3.4 (and now 3.5) for development and couldn't been any happier. The async features, specifically, are amazing.
When starting a new project from scratch, I see no reason NOT to use python 3. Maybe if a few libs you depend heavily on are still not ported.
Even in that case, though, it might be a good idea to start on Python 3. First because most libraries are already ported[1], and second, you could try porting that specific lib, or maybe running it as a separate service using python 2.
I guess the problem remains about maintaining big, Python 2-based programs.
When starting a new project from scratch, I see no reason NOT to use python 3. Maybe if a few libs you depend heavily on are still not ported.
Even in that case, though, it might be a good idea to start on Python 3. First because most libraries are already ported[1], and second, you could try porting that specific lib, or maybe running it as a separate service using python 2.
I guess the problem remains about maintaining big, Python 2-based programs.
[1] - https://python3wos.appspot.com/