I'm using the Classic.

I personally like themes that have a huge contrast between the read and unread emails.

i'm using the planets theme :P

Classic. After using Gmail for 4 years, I don't want to change it too much.

Night Shade :)

classic, I am still using classic. I like the themes very much but wonder if they sap my tiny bandwidth..?

Night Shade

Steel

Tree
....because I like having the weather there :P

I am going to switch back one day or another to classic, but since the release I have been using the terminal one.

Tea house ... its my favourite since IGoogle...

"Desk"....Love It!

Tree.
Same reason as Martin.

I use the default, and sometimes I switch to Mountains.

I use GMail a lot of time, so I need an uncluttered theme. I currently use Classic.
Phantasea looks great, but I don't like how the inbox looks. Pebbles is pretty, but it's too noisy. Desk is very cool, but it's heavy, and I need something lightweight to load fast and don't make my processor go crazy.
Minimalist is great too, but so gray for me.
So after a travel through many themes, I'm back with the classic look.

David Mulder
I would use the terminal theme, but in the UK (and Germany) the logo doesn't match the theme and doesn't look very good.

The terminal theme was interesting but the lack of 3D borders on form elements indicating "clickable" or "writeable" wasn't very usable IMO. I'm back to the classic theme, I didn't like the thick border of the new default theme for now...

I use Classic

Tree, because the pastel colors and overall color coordination and style is soft, clean, gentle, graceful... and shows weather effects!

No theme for me on gmail apps, I can't find any theme menu on setting my gmail app.

Tree.

I love the tree on the footer.

Terminal

It hurts my eyes, is glitchy (white background where it's unexpected, like Chat text) and is difficult to use. All in all, an accurate representation of the technology, reminds me of using XTerm in the late 90s, and actual terminals before that.