I primarily use Mac OS X 10.5 (Leopard) and Ubuntu 10.04 for daily use and Seriously I don't miss Windows at all, I use it mainly for academic programming, assignment and web surfing. I do gaming on Xbox and PS3 therefore I don't need Windows PC.
My both laptops Macbook(White, 2007) and Lenovo X61s(Dec'08) start crying if I try to run Win Vista/ 7 though both are Core2Duo with 4MB cache and 2 GB RAM.
I used to work on C# Asp tech when in previous Univ and previous work place but in California hardly anyone is into MS technologies and everyone uses Open source tools such as Eclipse, Komodo edit, Python, Java, GCC, GDB which forced me to format my PC based Lenovo to install Ubuntu as host OS and Win XP as guest OS using VMWare Workstation and it's amazing to see how both OS work so smooth while Win 7 cries to run smoothly on my Lenovo laptop.
I really like Mac since it solves most of my purpose except I use Mac MS office software for office related task but yeah Ubuntu in it's current avatar is seriously challenging Mac. Mac is secure since it's foundation is Unix/ Debian/FreeBSD so it's pretty secure from OS pt of view. Plus I am also in iPhone App dev so I need Mac.
Windows is flawed, on notebooks using antivirus software makes the performance very bad.