I
really like the new GUIs; they're great! I won't start a new game without them
Some comments on the GUIs:
Abysia: Great! Might look better with a border, though.
Atlantis: Great!
Pythium: Perfect!
Man: Great!
Ulm: I like it, but it's a little high-tech, and a little busy. Makes me feel like I'm playing Total Annihilation on Metal Heck.
C'tis: Perfect for Desert Tombs. It's probably not possible to do a different gui for the Desert and Swamp themes, though
Arco: Very pretty. A sky theme is appropriate, but... if it was possible to change the text to white (which it probably isn't) then a night-sky starscape would be perfect. However, if the text is black, that's not possible.
Caelum: Really nice! It looks a little bit "blue" and watery, though. I brightened it (increased gamma, brightness, and 'green' level), and it looks more icy.
Ermor: Very thematic, but... a bit too busy for me
Marignon: Nice, though a bit plain.
Pangaea: Looks a bit like green marble. Something a bit more 'foresty' might be better - with leaves and / or bark, for example...
Vanheim: Very nice! A little dark, though (for the text); I thought it looked better with an increased gamma, brightness, and blue level (makes it look colder).
Jotunhiem: Very nice!
Ryleh: Nice, but needs more tentacles. Just kidding
Mictlan: Nice! Dark like blood, yet does not reduce legibility.
Tien Chi: Very Oriental... BUT... it really interferes with reading the text.
Machaka: Thematic, but also a little too busy (hard to read the text).
These are just my opinions, of course, so don't feel compelled to change anything if you like it as-is