Ultimately this is more of a usability issue than a compatibility problem. But it's definitely a hassle.
Early adopters: You have been warned!
Desktop users will likely have fewer problems upgrading to Windows 8 than laptop users. That's because laptop manufacturers tend to maintain tight control over driver releases. Graphics hardware could be particularly problematic. Even recent-generation laptops with the latest GPUs may run older driversindeed, laptop manufacturers are notorious for their slow deployment of the latest drivers.
Consider the plight of one Windows 8 Preview Release user who owns a Dell laptop equipped with a very recent Radeon HD 7600M GPU: According to a post on Microsoft's support site, Windows 8 won't load the Radeon driver properly, and there's no updated driver on the Dell website.
It's possible that such drivers will show up on Dell's site on October 26, but it's just as likely that some delay will ensue. So even if you buy a new laptop that runs Windows 7 but comes with a $15 Windows 8 upgrade coupon, you might want to hold off on installing the upgrade until you can confirm that drivers are available.
Microsoft insists that current Windows 7 drivers will work on Windows 8. Nevertheless, a user will be presented with a message saying that the software has not been digitally signed for Windows 8 (this is true for every device maker that doesn't yet have a Windows 8 driver). Our advice? Take Microsoft's Windows 8 reverse-compatibility claims with a grain of salt.
The upgrade from Windows 7 to Windows 8 will be smoother than the one many users experienced during the Vista debacle. But no OS upgrade is trouble-free for everyone.
(PCWorld Senior Editor Loyd Case contributed to this report.)