I recently put Vista and Office 2007 on a new PC. I wouldn't say I'm over the moon, but Office 2007 is OK -- maybe a lot better when I'm more used to the interface. Vista certainly starts up much faster then XP, but I haven't been able to install some of my favourite software yet ços it doesn't work on Vista.
One thing I do appreciate though: I was working on a seriously huge workbook with a client (it really should have been in a database but he didn't have one), and his version of Excel had trouble even opening the file. Excel 2007 can address a lot more memory, and I was able to get the job done on my machine.
So, horses for courses. For the next few years there will be a changeover to the new formats. Maybe get it when you have the need to work on something in the new format, or you find yourself working with large datasets that exceed previous row limits. Otherwise if you have XP / 2003, stick with them for a while longer.
Denis