I've just started upgrading my office desktop to Vista. The current preferred route in our office is a fresh install. So, once Vista is on your machine, you have to install all those apps you've collected over the past two years :-( This is a nice way to waste a day I've discovered.
Overall, Vista seems quite nice. Certainly the UI is fresher. I quite like the transparent windows. The aero 3d menus are cute, but I doubt I'll use them other than to show off my graphics card to colleagues! Better is the new ALT+TAB interface. But overall I think MS has done a nice job of tidying up the windows experience - it seems more coherent than, say, XP. [Whether it's better in this sense than MacOS X is another matter!]
The problem comes with installing non-MS software - in particular some open source software. I rely on emacs, miktex, auctex and (to a lesser extent) tortoiseSVN to do my work. Here's where the problems are. Vista imposes a new security model on applications, which many now do not adhere to. So, for the time being, the apps that I need are very clunky. I found problems with all of the above. The solution for emacs/miktex/auctex seems to be to install some older versions. For tortoiseSVN I now use this on an XP machine only. Hunting around the web, it seems that Vista-compliant versions should appear sometime later this year.
I wonder if less MS-friendly people than me will be quite so tolerant?! I understand the thinking, but requiring app writers to re-architect code is going to cheese off a lot of people.