I like BeBerry, personally. It has messaging and calendar previews on the main screen, a special spot for your weather icon, and hidden Quicklaunch access. Slows down my Storm a little more than it's predecessor PreBerry, but still nothing that will make me switch back.
I've tried a ton of themes on various OSes. By far, the best combo has been .151 and Miro. Very fast, well skinned and a ton of extras. It's like having a new phone. I can hardly put my storm down now.
Posted from my CrackBerry at wapforums.crackberry.com