I recently updated to WordPress 2.5 using the most recent nightly version of the K2 theme. Everything went smooth as usual. I small glitch, when I select a theme version to use, I initially get a fatal error when reloading due to my last.fm plugin. I just refresh the page and everyrhing works great from then on.
The new look of WordPress 2.5 is very cool and has some welcomed refined touches/changes/advancements.
