Early this summer, I had a customer who bought a notebook PC (an HP) which happened to come pre-installed with Windows Vista on it.  Business requirements demanded that it should have Windows XP Professional instead, so I offered to 'downgrade' (ha ha) to XP Pro for him. 

As I learned (after hours scouring the Internet and chatting with HP technical support), this particular notebook _can't_ have XP Professional installed, because you can't disable 'Native Mode' for SATA in the system BIOS, and you can't install XP while the SATA drive is in native mode. 

That was the third strike for me.  I've had two other bad experiences with Vista 'upgrades', and have lost a good deal of time and money trying to work through it.  I've been a Microsoft Partner for several years, but I just can't support (pun intended) a system that is _so_ broken.  I refuse to recommend Windows Vista to any customers who rely on their computers for more than very casual use. 

Maybe Vista Service Pack 1 will be more useable.