Well,
I have used all of the Windows products from the beginning and can never remember a time when I have been more frustrated with an operating system!
A 'feature' of Vista is the 'User Access Control' which is a security measure introduced with Vista. There are dire warnings about switching it off - allowing viruses to enter the machine etc., but it is an absolute pain.
Every time you want to open a program that Microsoft do not want to recognise - like Firefox and some of their own products like Windows Fax and Scan, a window pops up asking if yuou are sure that you want to continue. In fact there are more pop ups with many things that you try to do.
If you want to connect the Vista computer to a network so that you can access devices and computers there, then there will probably be issues - many of them unresolvable because drivers are unavailable. I have a new Lexmark printer and cannot connect to it over the network from the Vista machine. Lexmark printers are not alone and there are lots of printers that will not connect. Access to the Vista machine will be difficult if not impossible without delving into the tangled depths of the security 'features'.
I also have a USB memory stick that I use with every other computer other than the Vista machine. The Vista machine will not recognise it. I bought it in mid-2007.
I was fortunate to attend a Vista release show and all of the emphasis was on security.. particularly for business not ease of use or reliability. The product is clearly aimed at this sector where the 'boss' may want to fully control his PC's so that even USB drives cannot be used, no software can be added, internet access is limited and severely controlled. Machines can be locked down so tightly they will not allow any flexibility.. like adding a new printer without bringing in a tech guy.
Forget the Aero interface - it is 'cute' but you will tire of using it and eventually forget it.
I also do not like the new 'Start' menu where it can be difficult to find the program you need. Unlike XP the list of program files does not expand across the screen to show them all. Vista has a scrollable window which shows some programs and other folders with programs where further delving is necessary. More work than XP.
Note that I am not plugging Linux... but the disk space overhead and the slow boot up and shut down times should make you look in that direction also if you must change....