the interface is (almost) exactly the same as F8's. Not sure if that's a good thing though.
That's because it's an alpha release. It's basically just Flash 8 with AS3 support. Other changes will be there in the final release.

As for the speed increase, keep in mind that it's only AS that's faster - the rendering-engine didn't improve that much as far as I know. Well, using the new Sprite class would be a bit faster than movieclips, but I'm not sure how much.