Just after the installation process, I was greeted by the Aero-based desktop; it turns out Windows decided that my machine was powerful enough to run it. It is true that the new interface has many visual pleasing effects (window shadows, fade ins and outs, a full screen task switcher...) but it is not that similar to Mac OS X as some seem to say.
Things as the control panel or the file browser have changed almost completely, in a way that it is trivial to spot dialogs that remain unchanged since XP. I like the new structure but you will need some time to get used to it; some things — specially configuration panels — have changed too much to be remotely familiar.
Then there is this new annoying security feature that warns you about every single administrative task you perform and asks for your permission to continue. It seems as a good feature to prevent intrusions from malicious software, but it asks so much questions that the regular user will end up answering without reading (if they are not scared enough to answer, that is). I bet they'd have organized this whole thing in some other way to minimize the amount of questions raised without removing them.
Overall, I would like to mention that it behaves quite well and, in my opinnion, it will be a good desktop OS. Unfortunately, some things are desperately slow like, for example, navigating your hard disk while there are other Explorer windows open. And, well... it is not Unix-based, something that could be really helpful for us programmers. I'll stick to NetBSD for that :-)
Oh, and given that this is a free beta I have already filled some bug reports in exchange. Now... it's time for me to leave for this semester's first final exam...