I’ve been running 3.4 on my test blog which I use for my plugins for quite a while now and am impressed by the sleek interface.
It’s not just the look that has been enhanced. If you like to test out new themes, 3.4 brings in the theme customizer which allows you to play around with various looks and settings for your current theme or one you’re thinking about switching to without publishing those changes to the whole world. For themes that support it, you can change colors, backgrounds, and of course custom image headers.
Also check out WP Candy’s Feature breakdown of WordPress.