So they released Vista when, to be honest, it was not ready.
People found old programs that worked under XP did not work under Vista.
People found that old hardware like Printers and so on did not have drivers for Vista so they did not work.
People found that if they tried to upgrade from XP to Vista they got all sorts of problems (crashes, devices not working and so on).
People found that if they DID manage to upgrade to Vista (or bought it on a new PC) it ran VERY slowly. So people had to buy more memory or a faster processor.
Since the release Vista has had a lot of bad press.
No doubt Microsoft have developers everywhere fixing bugs and finishing off the code.
My guess is they will release a Vista Service Pack 1 at some point, and then refresh the code, so anyone who buys it from then on will get the refreshed version.
Gradually software companies are releasing versions of their software that runs under Vista (or providing an update that makes it work on Vista).
But their are still many hardware devices that DONT have Vista drivers and probably never will have.
My advice:
I think anyone running XP should NOT think about upgrading to Vista. It is a lot of hassle, may not work, and you have no way of going back without a complete re-install of Windows XP.
A new machine is more difficult.
Getting a new machine with XP on it is getting more difficult, so you may HAVE to go with Vista.
But you may find that some of your old software and hardware may not work, so you may have extra expense buying a new printer for example.
If you can wait till Microsoft release a Service Pack 1 for Vista that would be a good time to buy. Wait till the PC or laptop you want to buy has a Version of Vista on it that INCLUDES Service Pack 1.