I'm so glad to have been involved in the development of Windows Vista, it's a wonderful OS. For the longest time I hung on to Windows XP SP2, thinking it's "good enough" but after using Vista for over a year now on my daily laptop, I simply can't go back. I always had Longhorn or Vista on machines in my office, but for a long time I had XP on my laptop.
I have a poster on my office door:
Is Windows Vista perfect? er, no! Will it have security bugs? Yes. But I really think the operating system raises the security bar substantially, and does so in a way that is pretty transparent to customers, and that's critical. Just about every defense I can think of, from image randomization, integrity levels, users-are-users, stack randomization, heap randomization, NX, exception handling protection, stack and heap overrun detection, IE7 defenses and ActiveX opt-in and so on are all enabled by default.
Again, Windows Vista is a pleasure to use, and from a security perspective, it'll be interesting to see how the security vulns pan out over the next, say, two years. I think we'll see real progress.