In general, I like the look and feel of the new UI. However, there are some significant issues. The two main ones for me are the borders on the windows and the title bars of the windows.
Borders:
The old UI had elements of the windows pretty much up to the edge. Photon puts some space around the edge. This is not inherently a bad thing, but nothing else was adjusted to compensate. For example, the buttons around the ship circle in the Fitting window are now half buried under the ship because the Fitting window was not resized at all (and cannot be drag resized) to compensate for the borders. Similarly, the New Mail window was not adjusted, and now the Subject text box is literally half covered by the Body text box (This might be related to the title bars of the window as well). The Mailing List Management window is also unusable for similar issues.
Title Bars and other sizing things:
The title bars and other ‘top of the window’ things on many of the windows are just too big. Particularly in the Fleet window (compact mode helped, but not enough, IMO) and the various inventory windows. I lost so much visibility of items in the inventories just from switching to Photon, that I had to enlarge the windows to see half of what I could see before. The icons of things might also be larger, but I’m not certain. But the title bar and top few ‘static’ info things at the top of the inventory windows take up far too much space. And ‘Compact Mode’ does not at all help with this, because it completely changes the layout of these windows (it hides the ‘hierarchy view’, among other things).
Overall, I like the feel / ‘visual’ changes of the new interface, but I don’t like the ‘content’ / ‘layout’ changes. Too many windows lost valuable realestate to ‘useless’ things being enlarged. It may have been an ‘eye chart’ before, but at least you didn’t need a 42 inch monitor to be able to fit everything you need to see on it at once.