I spent about an hour or so just now upgrading and tweaking WordPress 2.0 and so far I have a very good impression of it. It definitely has an AJAX flavor to it now — admin pages are more customizable and features just flow nicer.
One of the more noticeable differences I can see is the restoration of the Dashboard. In version 1.5.2, I could only see the right side of the Dashboard — the left column was mysteriously missing. Yet in another install of 1.5.2 that I had done, the left column worked just fine. However, the Developer blog entries are missing from the Dashboard now! Such a pity.