Basically, the UI is shiny but awful - really, really clunky.
You can't compose content within the view of your own blog. Settings are kind of strewn about the dashboard and blog view. The themes are poorly organized, a lot of them have broken links outright (!), and in general, there are just a lot of WTF moments with the interface.
Sure, everything's glossy and animated, but use it for any length of time and you get the distinct feeling that it's a hack.
Siri was an empty vessel into which we poured our hopes and dreams for more personable computing. Too bad it turned out to be a flaky frontend for Wolfram Alpha.