Well, if you allow me to give my opinion:
Most of what is said above is true and Vista without SP1 sux big time. It is slow, clunky and many operations that are bread and butter of an operating system (file copy, for example) are terribly slow to the point of being unusable.
SP1 brings a new kernel and lots of changes to the operating system. If you used Vista before SP1 you will notice the speed improvements in certain areas. It also reduces the number of security confirmations for some operations (some operations required you to confirm 4 dialog boxes!!! and now require 1). If you want my honest opinion, XP is a more rewarding operating system as it will let you do what you want and not try to be as "nanny" as Vista.
Albeit using Microsoft technologies for work and being seen by many people as a die-hard Microsoft fan I got fed up with Vista and bought myself a MacBook Pro which I just love. After that my Vista laptop gets booted less than once every 2 months.
I still use Windows XP/Office 2007/Visual Studio 2008 on my work desktop though.
If you want my advice, Microsoft just released Office 2008 for Mac that comes with Word, Excel, PowerPoint and Entourage (half-arsed Outlook replacement). It uses the same file formats as Office 2007 and depending on your needs will do just fine. The Mac needs several less reboots, updates, security patches and security confirmations (none) than Windows and just works quite nicely.
Also, you can install Windows XP or Vista in a VMWare Fusion virtual machine in the Mac for when you need Windows. I have created a XP VM on mine on day 1 and I use it only when I really need to do some work in Visual Studio from home, otherwise I never turn it on.
I also have been using Linux since 1999 and although I like it very much I wouldn't advice it to someone that doesn't want to spend some time fiddling with it. If you don't have 100% supported hardware it may simply not work or you will have to spend a lot of time getting it to do what you want. An example is that the Dell laptop where I ran Fedora 8 (RedHat's open source Linux) uses a Dell wireless card with a Broadcom chipset that is not supported, so you install Linux and don't get wireless network. Because there's no device driver you can just download from somewhere and install you have to fiddle with the Windows drivers and a program called NDisWrapper and try to get it going.