I like the idea of GMail Themes more than the initial batch of available themes (though I am sure more are coming). While there are several "cool" themes, my eyes can only tolerate a couple of the color combinations. That said, there are little tweaks that are as impressive as they are fun. The Ninja theme is too colorful and hectic for daily use - but the attention to small details is great. Look at the GTalk pop-up boxes and how the edges are mildly raised / angled. Small touches like that have be strangely excited for the eventual wave of new themes. Finally, why does Google ask for your location (city / country) when you choose a theme? Is there a real reason here or is it for something sneaky (answer is likely both).

GMail Themes