Well, I will be thankful for making it easier to change the admin UI, it's really great, I appreciate this "less pixel graphic" approach. But, frankly, "new" UX is still the same, without any improvement, while new UI is just awful: inconsistent, low-contrast in important details (excessive contrast between menu and working area and totally lost one for submenus, headings, metadata, fonts in general, etc), utterly ugly icons, random spacings...
Does it look trendy and "fresh"? Yes.
Does it look as a convenient tool for the content management? NO!
WordPress is no longer "just for blogs", don't you know? Many people use it to manage complex multi data web solutions. We have long been in need of intelligent re-engineering of WP admin, of more sophisticated (but clear and convenient) settings for users. Instead, we just have an ugly UI re-design.