It's a whole new OS. It's not just a facelift. This is both good and bad news.
The bad first: you'll need new drivers for all your hardware, so you're at the mercy of hardware manufactures to write then. Also, a lot of your software will need to be updated or patched. The GUI is different enough to make life tricky for a few days, but then you're over it.
The good news: Security. It's a lot better, especially if you bear with the User Account Control (UAC) and don't just turn it off like some people
Aero is pretty, but you don't really notice it after a while. The new Media Center is nice, but doesn't offer much new functionality. The DirectX 10 games are going to be spectacular, but will require serious (and probably new) graphics hardware.
For business users there's a lot more new features which are useful.
In summary: There is no compelling reason to upgrade. If you are buying a new PC then Vista is a no-brainer. In any case you need to be prepared for the changes and live with the short-term pain it brings.
I've been using Vista full-time since October, and I'd never go back now.