My Gmail account was finally updated to use the new themes earlier today (I need to be put on whatever server gets upgrades quickly). I looked through all of the themes, but have stuck with the Ninja’s theme for now.
You would not think that a ninja theme would make things easier to see, you might actually think a ninja theme would make things blend in more (much like the ninja does itself). However, one of the things I really like about this theme is how well the sidebar items are set out from the rest of the page. With the old theme, the sidebar items blended in a bit too much. It is super easy to see the items on the sidebar now.
Of course, the Ninja rocks as well.