Perhaps this is the most common question everyone is asking after Microsoft announced the new OS and force all of us to use it. So you may want to justify yourself, why should I buy or upgrade to Windows Vista? If you want to do a comparison between Vista and XP, just google it and you will find plenty of information. But, don't waste your time because all these comparisons are just crap.
Here are my advice:
If You're Windows XP Users
You don't do anything. You don't need to upgrade to Windows Vista. Unless you really have a very valid reasons to upgrade. Do you? Perhaps I'm wrong? In terms of usage of my personal experience, I don't feel any major differences between XP and Vista.
Their interface is exactly the same except those fancy UI which I normally disable them in both XP and Vista for performance. Honestly, I don't feel there is a need to enable all these fancy UI stuff? Other than just make myself pretent to be cool at Starbuck, I really can't find a single reason why we need those fancy UI stuffs. So the Windows Aero that introduced by Vista is completely useless (my personal opinion only since I don't turn it on). It looks cool especially the 3D effects, something like Macintosh UI, but it hurt the performance so much. Stick with XP is the best unless you really don't care about performance and you love fancy UI. Perhaps you want to consider Macintosh?
One more last Vista feature I like (yes, this is the only one) is the security features - User Account Control. Even if you have the full access to you PC, you're required to allow the access as an administrator for any application trying to access your hard drive. This is something like (but not exactly) in the Unix environment where normally people do not login as root and you're required the root password to access root's files and folders. This gives me a peace of mind that I do not need to worry any intruders, hackers, or viruses anymore. A pop up message something like below appears whenever there is application trying to access your PC.
User Account Control is the one Vista feature that I want, just because of only one feature should I upgrade to Vista? Have you ever thought why Microsoft want to invent (does they really invent) the new OS? Why don't just integrate as part of the XP update? I think you should have already had the answers for the question. If I were to buy Vista, only for one reason - the User Account Control features. Nothing else.
If You're New PC Buyers
You're buying a new machine and thinking to whether to buy XP and Vista. My advice to you is to buy Windows Vista. Not because it is good, it is because we're being forced. What else can we do? I don't think you want to spend your extra money just to buy Vista again when Microsoft announce XP will be obsolete few years from now.
In conclusion, buy Vista if you do not have the existing OS. You do not do anything if you're existing XP users until they obsolete your baby XP one day. Then, at that point in time, you may want to consider the upgrade. By the time, I believe the Vista should already more stable than today. Having say so if you really like fancy UI stuff, buy Apple Macintosh.