Just like everyone else these days, I am jumping on the Drupal 7 bandwagon for my newer projects. And every time, I come across the question which theme to use. By default, I gravitate towards Zen. I know it inside out, it is still the most-used theme across all Drupal sites and it just seems like the right way to go. I'm not a big fan of splitting up my css into tens of stylesheets, but I can go with the flow.
Then there is Fusion, which is "a simplified 960px" grid theme and #2 on Drupal.org. As a developer, I often come across these red flags that should be slightly orange ones at best, but I just can't stop including them into my decision making process. With Fusion, that would be the fact that it is a theme by "Top Notch Themes". Nothing wrong with that company, but it's just that: there's a company behind it that sells most of their other themes. Other than that, it seems to be a perfectly fine theme.
And then there is the new kid on the block: Omega. With a cool name like that, how could you say no (even cooler: the core and starterkit themes are called Alpha and Omega => geek points for that!). Something that I need for my projects is to have most of the configuration to live in the code, not the database, for a lot of reasons other than "versionability". And from some developer friends, I've heard that Omega's configuration lives mostly in the database. That being said: the usage statistics for Drupal 7 are crazy good: http://drupal.org/project/usage/omega.
The theme package with the fanciest HTML 5 logo is AdaptiveTheme and its numerous sub-themes and sub-themes thereof.
Enough said, let's get to the trial & switch-to-another-theme part :)