Used to use the 12wx in class. It's pretty nice, though the big power cord can get pretty cumbersome, and working on a 12-in screen can put some strain on the eyes. Expect to be doing a ton of scrolling/zooming too, especially if you work with large canvases.
If you're really serious about drawing, get the 21" or larger.
They're not really lap devices or portable. The screen being that small makes it an excellent overpriced sketchbook because it has all the pressure sensitivity expected from an intuos - but the screen real estate makes it a pain. At least with a tablet you can map it to a large or small monitor.
Well good luck with that because both models of Cintiqs can be picky about your graphics card and can affect performance. But you've already convinced yourself and went out and bought the small one anyways. I mean this especially if you're using this with a laptop.
Invest in a screen protector. Even if you can live with the fact that you'll get some small scratches based on regular use, they can get the stylus to skip around when you hit them and mess up a line if you're trying to make it nice and smooth.
Definitely play around with the mapping for the buttons and touch sliders. There are some really useful things you can get them to do.
Be aware that some art programs don't like the Cintiq. OpenCanvas refuses to recognize it, and Gimp likes to get confused about the sensitivity sometimes, or at least 2.4 did. I can't speak for any of the other non-Adobe products.
Try to keep the screen clean. That's probably a given, but you'll be rubbing your hand/arm all over it and it's going to get pretty smudged, which can affect how easily the stylus moves around it. Also, the salt from sweat can help cause scratches.
And just generally treat it like any other thousand-dollar piece of technology, I guess.