I only had about an hour to play around with it last night, as I had to be up early this morning - however, here are my impressions...
At the moment, things do seem to be a little slower, however I'm thinking because this is a beta, things will improve... I have faith!
It is, however, an improvement over Vista, which is always a good thing.
In my hour of playing with it, I installed FSX (As you may know, I had problems with it in Vista where the ATC window was tiny), I got on it, ramped the settings to max and started it with no problems whatsoever! The improvement in frame-rate really impressed me, and the fact the ATC window has seem to have cured itself, is a nice bonus!
One thing I will have to get used to is the small icons in the taskbar, instead of full tabs - however I did like the fact that when you are downloading something in IE8(Beta), the download bar is shown in the small tab in the taskbar, which means you don't need to switch to the download window to see the progress!
Anyways - I hope to play around with it some more tonight after work, so hopefully I shall be back
Marky