The UI is interesting. The comparaison with WP7/8 is only relevant because of the dynamic aspect of the tiles. However, while it is with WP only informative, the real innovation from Jolla is to make the interactive, which can be very useful (the audio player example is obvious but there are for sure many applications like the phone torch).

Obviously, a lot of design work is missing to make the OS visually efficient, especially to give a solid impression when presenting the UI (better contrasts, better icons, better fonts, etc...) but that will probably be improved before the official release.

The major problem is in my opinion that the transition effects are not intuitive enough, especially when starting applications and swiping back to the home screen. The harmattan solution is very natural but there are for sure many other ideas to explore (like scaling up/down the application from/to a tile). The fading effect of Jolla UI is mostly confusing.

I really like the way they simplified the "home screen" to integrate both multi-tasking and the 4 main launchers. The vertical nagivation however (from the lock screen to the application grid) is in my opinion the wrong choice as it makes the path longer, at least for mobiles (because the height is usually much larger than the width...). Another problem is to mix the home screen and the application grid in one layer. It is neither obvious nor convenient to use. Maybe a button and a simple modal frame would be more efficient.

All in one, a quite good UX concept, especially the way they make the tiles really dynamic, but which still need some polishing.