On one hand, it worked quite nicely perched on the desk in front of my mammoth CRT, though that required pushing the keyboard off to the side, within reach for keyboard shortcuts. It didn't function quite so well on my lap, however. Without the ability to adjust the angle it from the top (to tilt it toward me), I couldn't find a comfortable position in which to work for more than a few minutes.
In a multiple-monitor setup, you can program a button to jump the cursor from your current application to the desktop, or from the current application to the application on the other display. When working with the other display, the Cintiq functions just like a tablet and supports all the functions that the Intuos3 series offers, including pop-up menus, a programmable switch on the pen, and customizable pressure sensitivity for the pen and the eraser.
Configuring the buttons is easy; remembering what you programmed is hard. I generally had to leave the Tablet Properties dialog open on the other display for "quick" reference. Furthermore, I had to disable the Touch Strip on the right (I'm right-handed); it proved impossible to keep from brushing along it, causing unwanted actions.
One frequent misperception about the Cintiq is that it's somehow a lesser tablet, since it does double duty as a monitor. That's untrue. The 12WX is based on the most recent of Wacom's tablet technologies, the Intuos3, with its 5,080-line-per-inch resolution and 10-bit (1,024 levels) pressure sensitivity. It supports the same set of wireless pens: one standard Grip Pen comes bundled, plus there are the optional 6D Art Pen ($69.95), Airbrush ($99.95), and Classic Pen ($49.95). It doesn't support the Wacom mouse, though I can't see any reason why it should. And in practice, it certainly feels the same as working with an Intuos3 tablet--better, even, once you factor in the more natural feel of working directly on the display, especially with brushes.
I've got mixed feelings about the 12WX as a display. On one hand, the device itself is the perfect size when you don't have a lot of desk space and don't live in your graphics applications full time. The screen is too small, however; I would have preferred it with a 4:3 aspect ratio and smaller buttons in order to provide a bigger active area. Creating masks in Photoshop, for example, required too much zooming around. I also found it too difficult to operate interface items along the screen edges, such as scroll bars and the zoom pop-up on Photoshop's status bar.
All that said, I really liked working with the Cintiq 12WX and will be sorry to see it go. If you've got the budget and the need, there's no reason to deprive yourself; if you've got the space, you may want to consider one of the bigger options, as well.