Recently I decided to upgrade my Windows XP Pro system (a Dell XPS 410, purchased last October) to Windows Vista; I had been holding off for awhile, but was curious to get going with tne new O/S. The reason I chose the "Ultimate" version was so that I could do the "in place" uprgade, which means that you can install Vista over your current O/S and programs; most verstions of Vista, including the "upgrade" packages, by default do a "clean" installation, which means it will delete your current O/S, all your data, and install a fresh copy of Windows.
Well, I found out, that I should have done what I wanted to avoid, and taken the time to do the "clean" install, and reinstall my programs. This is why almost everyone will have problems with some of their programs after the upgrade: Vista moves various system and data files from your \DOUCUMENTS AND SETTINGS\USER folders, including, your \My Documents and all sub folders, \Local Settings, which contain your \Application Data and \Temp folders, and many others. Thus when you restart after the upgrade, if the program you have installed had files there, and the Vista Setup script didn't know to move them to your new storage locations which is \USERS\USERNAME\DOCUMENTS or \USERS\USERNAME\APPDATA, that/those program(s) will have problems finding their files and settings (including older versions of Microsoft Outlook not being able to find its .pst files, LOL). This aggravated me because it took HOURS to figure out; Microsoft should provide printed doucmentation something to the effect of "To Advanced Users-the Vista In Place upgrade will move these files," etc and how to deal with it. This would save a HUGE amount of time. Less advanced users probably simply assume their system is broken and roll back the upgrade.
Once the O/S is installed and troubleshooting is done, I think it's excellent. After about 15 years, it's actually more document centric than program centric, e.g. instead of "run" on the start menu, you have "search;" you start typing in a text box and the start menu shows you everything-files, documents, web pages, emails, whatever that have a matching text string. So for example instead of launching Word, then looking for and opening the novel im working on called "Silver Lining," i type "Sil" into the "search" box, pick the "Silver Lining.doc" entry, and Word launches and loads my document. Finally more like the Mac than previous Windows, which were really basically DOS with a graphic shell pasted on. (Yes I know you could already launch programs with documents, but this is more elegant, you'll see what I mean after you've got to do it a few times).
I'm quite satisfied with Vista; in fact with my late model hardware it's like having a new computer again. But, my advice to most people is, if your computer is more than about a year old, wait for a new machine. If you are upgrading a recent machine that has the hardware needed, back up all your personal data, and do the clean install. Changing operating systems is not for beginners; if you have any doubts, it's best to pass.