I upgraded Chaos Laboratory blogs to WordPress v2.1 just sometime back. The upgrade happened without a single hiccup and to my surprise each and every one of the old plug-ins worked perfectly without breaking the installation. Even my old theme worked at one go - without any kind of modding.
I love every bit of the new interface and the features 
Terrific work on this one, guys !! Specially the enhancements on the TinyMCE editor, which allows you single-click switch between WYSIWYG and Code views. And the auto-saving of drafts is just oh-so-cool.
Ah well… I better stop now or else I’d be on and on with my incessant ramblings. It’s 101% worth the upgrade guys.. Go for it. 
