I've been using it for the past 15 minutes, and my opinions of it are...
...mixed.
Barring a couple of things here and there, I do like the new UI. The new multitasker is so, so much better than the previous one.
What I don't like is how it's made my iPad 3 chunkity, not quite as smooth, and occasionally unresponsive. Just changing wallpapers can sometimes lead to 5-6 second freeze ups. Other times I have to press buttons multiple times before I get a response.
So...
UI? Pretty alright. I haven't played with it enough yet, but I could see it growing on me.
Performance? Not impressed. At all.