I have a Toshiba M200 tablet PC and it is a very nice setup with ZBrush installed. But so is an Intouos tablet, a PC and ZBrush. The only real advantage is that you can easily take your tablet PC with you anywhere you go. Though I usually only reach as far as to a nice big comfortable chair 
There are differences in the tablet interface of a tablet PC and a standard PC with a Wacom tablet. Nearly all Tablet PCs use a Wacom interface for the pen but to keep the price down and possibly due to technical limitations it is older technology. That means on practically all tablet PCs you have 256 levels of pen pressure whereas on a standard Wacom usb tablet you have 512-1024. I have grown used to the 256 levels of pressure on my tablet PC but it might cause you problems if you are used to photoshoping at 1024. For editing a model in ZBrush 256 levels is quite sufficient or at the very least you can grow accustomed to it.
I also use a Belkin Nostromo 52 keypad together with the tablet PC. It is great for assigning shortcuts for various programs. It is a necessity for me when using the tablet PC and ZBrush,as I NEED my shortcut keys. See here for more on the Nostromo The Best 50$ you’ll ever spend.
The Centrino CPU on the Toshiba is very good. The same ZBrush scene rendered on my 2.8 GHz P4 and the 1.6GHz Centrino was only 10 seconds slower on the centrino. And the ZBrush HyperThreading benchmark speed was faster on the Centrino. It is a good CPU. But for heavy duty ZBrush work with millions of polygons, the faster ram of a desktop system will win out.
All in all I find it very enjoyable to doodle models in ZBrush and practice my ever lacking drawing skills in Photoshop. Is it a replacement for a desktop, no. Does it equal the Cintiq, no. But if you are looking for a laptop with a fair kick in it and that lovely tactile feel of drawing on a screen it is a good option. For me tablet PCs are great as they are small, as a laptop should be, don’t lack power (except for the graphics card, not important for ZBrush) and do everything I need to do with a pen interface.
Good luck
A tablet PC thread here on ZBC. Tablet Pc for Z-Brush ?
My mini review of the Toshiba M200 and ZBrush at CGtalk.
Also check out the forums at TabletPCBuzz. They have forums for each of the major Tablet PC manufacturers and user reviews of each tablet.
In order to use the pressure sensitivity of the pen in ZBrush you need to download a special driver from the Wacom site. This driver is needed for all non “pen enabled” programs (ZBrush, Photoshop, Painter, etc).
