The SIP support looks promising but I've been burned before when they announce something is implemented but is far from actually working in real life. See CalDAV or CardDAV. In the end I gave up and stuck my SIM in an Android phone. Having stuff sync reliably and decent multi-tasking (yes, on Android) outweighs the distinctly average UI on Android.

The UI changes are a mixed bag. The events view and carousel look like a move back to the N9 layout (good) but the rest are pretty terrible for a phone. Losing cover action gestures and adding android style status bars is awful. I don't understand why they'd ditch their uniqueness for the sake of a few stupid users who insist on seeing the time or battery level all the time.