It's nice to have more options without using external scripts, after all Hotmail and Yahoo! also have themes (the latter in a very limited way, though). The most complex ones look a bit flashy to me, but the just-different-colours ones are OK, so I'm giving them a whirl.
You've got to give props to Google for rolling them faster than usual, and worldwide as well. And by supporting them in all major browsers, about time GMail treated everyone as a first-class citizen :)