I've been testing Windows 8 on a few different tablet PCs. Specifically, I installed Windows 8 on the HP Slate 500 and the HP EliteBook 2760p Convertible Tablet PC.
In the case of the Slate 500: this device is really built to support Windows 7. HP does not provide Windows 8 drivers on their website. After installing Windows 8 on the Slate 500, I found that some key features didn't work. For example, the N-Trig pen does not work properly under Windows 8. There are some beta drivers that you can find on the N-Trig website, but these drivers don't support all the functions of the pen. I hunted around for a while and updated a few drivers, but I finally concluded that the HP Slate 500 is better off running Windows 7.
On the EliteBook 2760p Convertible Tablet PC, Windows 8 offers some really nice features and upgrades over Windows 7. In tablet mode, the touch experience is much better than Windows 7. The 2760p is a convertible tablet PC that supports both capacitive touch and an active digitizer pen. I didn't have any problems using touch or the pen. In touch mode, the on-screen keyboard is much easier to use. Overall performance feels great and Windows 8 runs very smoothly on the 2760p. Battery life on Windows 8 is supposed to be better, but I didn't spend too much time measuring that improvement.
Before upgrading to Windows 8, Microsoft recommends that you run the Windows 8 Upgrade Assistant. It scans your current PC to see if it is ready for Windows 8 and then provides a compatibility report and optional steps for you to buy, download, and install Windows 8.