This started as a day of Vista, but now I have been running it for a week, on my laptop and on my desktop. So far it is working without any major issues. One thing that is a little annoying is 120dpi support, I boosted up the dpi to make the screen easier to read 1920x1280 makes things a bit small. Some application look great at 120dpi other application look like zoomed in jpegs which doesn't help the readability very much, they probably would look better at their default sizes. The other annoyance with 120dpi mode is many websites specify their fonts using pixel sizes so they all stay small.
Some of the application effected by the weird scaled versions appearance are a bit surprising, like Windows Live Messenger, and Windows Media player. The one application that is scaled that will probably force me to switch back to 96dpi is VS.NET 2003
If you want to do fancy task switching use Windows+tab, not alt+tab. 
One cool new thing, I think it is new anyway, if you have the alt+tab pane open you can select the application you want with the mouse, this also works on the window+tab variant. If you run a lot of application at once like I do mousing could be quicker.
The security popups that you will see a lot of people complaining about are really not that bad. For the first few hours as you are installing drivers and tweaking things around you will get them a lot but after that you will rarely see out. Except if you want to check the calendar, you used to be able to right click the task bar and get a handy calendar, now you only get a clock. To get the calendar you need to click the second change datetime button, which gives you a security popup.
When I needed to print something on Saturday I was a bit worried as my printer was a bit of a pain to get working under XP, under Vista I just plugged the usb cable back in and it fired up right away I didn't need to do anything.
One new thing that I really like is that you can switch users when connected to a domain. I like being logged in all the time, but other people use my desktop sometimes, now I can give them their own logins again.
So far this is the best build yet, and the only one that I have been able to use for an extended period of time.