I used Vista beta right from February this year and later RC1, right until I got my Mac just a few months ago. Vista was one reason I got a Mac, which should say a lot.
With Windows XP, Microsoft finally built a decent OS for the home by adding a nice GUI and Direct X to NT...:headphone but by leaving all the doors open and insisting that IE could be controlled by everyone and anyone on the world wide web, old Redmond built a security nightmare.
A few years later and SP2 is what should have been delivered in 2001. However if you were a veteran, SP2 was simply an annoyance. 'Windows has detected you're not using an Anti-Virus programme'. 'Windows thinks the web site you're visiting has too many colours'. Etc
Now Vista is like that, only 10 times worse. Vista warns you about everything and anything, whether you're installing an update, deleting a file, moving a picture from one folder to another or daring to use a non-default media player.
On the positive side, Vista can look nice if you have the right hardware. You do need a gig of RAM but worse, of you want to run Aero in full you need a 128MB DX9 compatible GPU. On a laptop this is quite rare and gobbles up battery life anyway. I ran it on a laptop that was a AMD3000+ with 1.5gigs of RAM with a Radeon R9600m with 64MB of RAM. Once booted, it was surprisingly fast (for a Beta OS). I also ran it on a desktop, 3400+ with 1 gig, plus an nVidia FX5900u with 256MB of RAM and dual Sata 150 drives. It was quick.
I also installed Media player 11 (which astonishingly looks remarkably like Songbird... I don't know who copied who) and also Office 2007, which is the ugliest thing MS have ever made. In fact from the beta programme they apparently changed the interface so you could get the classic look back.
Vista will no doubt succeed and it is actually pretty customizable. But where OS X is simple to use and intuitive, Vista simpy patronizes you through wizards to do everything and, IMO, still looks cluttered. It's pretty huge, Windows Mail is a rip off of Mail, IE7 takes cues from Firefox and Opera and Aero Glass is 5 years late and has already been copied a dozen times.
It's a decent OS, but it won't be fun for a lot of people to use. I expect many more to follow into Apple's arms.