I had the chance to checkout the new user interface today, and here are my general thoughts on it overall.

TREMENDOUS START! Instantly, the game feels fresher, and more modern.

Text: AMAZING. So much easier to read, I cannot stress how much of an improvement this is for larger displays (HDTV).

Look and Feel: I think this needs a lot of work. I am assuming this is not the final appearance intended. The color scheme is a little strange, however the dark, clean, translucent background where text is displayed is fantastic, I would keep that part of it.

Viewing Equipment: Also, an excellent improvement, being able to see longer lists of items is wonderful.

Viewing selection dialog: A tremendous improvement, not having to scroll to view more than 3 options was a sorely needed improvement, and I'm delighted to see that this has been improved!!

General Usability notes: I couldn't figure out how to move certain elements like the dock, or macro pallettes (perhaps this isn't working correctly yet?). The submenu during battle feels a bit clunky when playing non-mage jobs, however it's extremely useful for mage jobs. For example:
I tried the new user interface in battle while playing Paladin. The selection of spells was much easier than the old UI, with large, clear icons, and the fact that the window didnt go away after casting one spell was very nice!!! However I found myself fumbling around when trying to switch between some job abilities and spells and weaponskills. I'm not sure how this could be improved at this point, perhaps it may just take some getting used to.

I can tell already I will love the new interface when playing jobs like Scholar, Bard, Geomancer, where I typically do not have all of my spells setup as macros.

As I play with it more I will post more detailed feedback on specific features, but I would like to say this:
ABSOLUTELY INCREDIBLE WORK SO FAR!!!!!!!!!!!!!!!!!!!!!!!!!!!
I love this game, and while things like battle mechanics, storylines, cut-scenes have evolved wonderfully over time, the user interface has been the one glaring thing that has not caught up with the times and makes the game feel very dated, especially in the transition to HD displays. This new user interface, while it needs some work, is a TREMENDOUS IMPROVEMENT ALREADY, that makes the game feel more modern, it makes text easier to read, and is an unexpected wonderful new direction. I'm looking forward to seeing how this progresses!