ROS Indigo is stable and ready for users. There haven't been many changes since Hydro, so upgrading should be fairly straightforward; see the Indigo migration guide for details.
That said, upgrading Ubuntu from 12.04 to 14.04 has been giving people trouble (Ubuntu migrations with third-party packages tend to). If you can, I recommend installing Ubuntu from scratch instead of upgrading. If you can't or don't want to do a full, clean install, I recommend removing all previous versions of ROS (from debs; you can keep your local workspace) and running an apt-get autoremove
BEFORE you upgrade Ubuntu. This should help bring the system back to a semi-stock state so that the upgrade can run cleanly.