Whenever MS brings out a new OS I always wait a few months or for the first service pack before I upgrade and I've never had any problems.
Windows XP was great, no nonsense, rock solid and familiar to anyone familiar with Windows NT and Windows 2000. Unfortunately, support for XP will end next year, so you might have problems finding drivers etc. if you want to install it on new hardware in the future: http://windows.microsoft.com/en-GB/windows/end-support-help
I waited for the first service pack and I had no problems with Windows Vista once I got used to where things were; I still have it running on a second "Media" PC.
Windows 7 is fine and operationally similar to Vista.
I had my first look at Windows 8 on a friends laptop the other day and I'm not in a hurry to upgrade from Windows 7. I'm sure I'd get used to the new interface and dragging aps to the bottom left corner to close them, but it's slower/clunkier than clicking a red X in the top right when using a keyboard and mouse - perhaps there are ways of making it work more like older versions that I didn't discover in the short time I played with it; in a similar way that you could make Vista and 7 look like "Windows Classic".