Update: Okay, we can neither log in nor get signed up at present, though others have clearly achieved the feat already. Twitter.com itself keeps alternating between its new and old versions, so we suspect there's quite a bit of work going on behind the scenes at present. To answer your queries, loading time from the live tile menu to the top tweets page above is approximately three seconds, while scrolling is basically identical to the perfection available on WP7's own apps. Swiping laterally gets you into Trends, Suggested, and Nearby categories which take a couple of moments to load up their tweets, but otherwise match the performance.
Exiting to the live tile menu throws you out of whatever you were doing and re-entering the app -- as is par for the Windows Phone 7 course right now -- means starting from scratch. The
Hold up, reader David Gordon points out that you can hit the live tile menu via the Windows/Start key and then return to Twitter through the Back button, yay, that works too.
Update 2: There are still errors being thrown up, but we finally got ourselves logged in. Well, it looks just like the vid promised it would and the app itself is working flawlessly, there are no processing delays that we can see. Oh, and there's a landscape mode. Our only bugbear is that there's no differentiation between your own tweets and those of your friends. Ah well, check out the gallery below.