I've personally found the upgrade process from Windows XP to Windows Vista to be seamless. However, I know that people have concerns about their software and devices working with a new version of the operating system.
In Windows Vista there is pro-active and reactive technology to help with compatibility issues, so I think compatibility problems will be not be as big a concern. However, with over 200 million users, there will be issues for some people. The best thing you can do is plan ahead.
Use the Windows Vista Upgrade Advisor on your current machine to identify potential problem areas. In many cases where there is compatibility issues, there is already a new version available from the manufacturer. The upgrade advisor will link directly there where you can get the new version.
For technical folks, you can browse the Hardware Compatibility List (HCL) and see the level of support available.
Now for a purely personal comment. In the past, I've found that software that doesn't work with newer versions of the operating system (not just Windows Vista), tend be of lower quality overall, or tend to perform tasks that require intertwining with the operating system. The days of allowing applications free rein to manipulate the file system and registry is over. Too many applications abused this and Windows has had to clamp down to prevent exploits.
Already have Windows Vista installed and are having problems? Here is some help.
Now, for Speech API developers, I'm happy to say that Windows Vista includes SAPI 5.3, which is backwards compatible with SAPI 5.1 that was included with Windows XP.