Windows 7 is a definite improvement over Vista in a few areas - the look is a bit more polished and it does run better on older and or slower hardware than does its predecessor - but it doesn't introduce any features that are absolutely "game changing" or "must have" for the majority of people.
That said, the writing is on the wall: it's not a question of "if" one should upgrade to Windows 7, it's a question of "when" to upgrade. XP is well past its prime. And Vista - having never really caught on like Microsoft had hoped - will likely be overlooked by most.
Ultimately, as with most things, it comes down to personal need and preference.
Do you need a new computer? If so, then your answer is simple. Any new computer you buy will come with Windows 7 pre-installed or give you the option to upgrade to it for little or no money. If you intend to build your own computer, you've most likely already decided which OS you want it to run and have planned accordingly.
If you're not in the market for a new computer, but are trying to decide whether upgrading from Windows XP or Vista on your current machine is worth it right now, ask yourself the following question:
Does my computer do what I need it to do?
If you are satisfied with your computer as it is now, why upgrade now? Windows XP will be supported by Microsoft until 2014, meaning they will continue releasing patches and security updates for it until then. Assuming the developers of your favorite programs and applications continue to support and maintain compatability with Windows XP, you should be fine for another few years at the very least. There's no reason at this point to believe that Microsoft would discontinue support for Vista before 2014, either.
And keep in mind that, in general, the longer you can get some good use out of your current setup, the more money you'll save in the long-term.
On the other hand, if you absolutely must be on the bleeding edge of technological advancement when it comes to your computer, well...you've probably upgraded to Windows 7 already!
Still as indecisive as ever? Give it 6 months and reevaluate. That will at least give you time to put some cash away for the inevitable purchase, and when you finally decide to upgrade, Windows 7 will have more patches and security updates under its belt, making it that much more secure and stable.