It's good for black and white stuff, because that's why I got it (to do my comics). The color profiling in it is still a bit screwy so if you intend to use it for color, be ready to take the final piece in your normal color-centered monitor and run a few Color Balance and Level patches over it at the end. With a bit of time you'll get the hang of where it's different (for example my cintiq I've estimated is 10% more saturated than my working monitor, and has a spike in the reds and yellows of about 5%).
But I use it for comics, and there, in combination with Manga Studio, it's really handy. If you check my blog you'll see a long-running comic I'm doing with Cintiq for inking. I do the pencil work by hand and scan in it, however. Let's put it this way: if I were doing this same comic by all by hand, it would look a bit worse in terms of detail and control, and it would take (I'm not kidding here) about double the time it takes to finish a page. You can't imagine how much time even a simple floofill takes. Or being able to work with WHITE on top of black, not having to think ahead and leave white detail out while you fill in blackness.
However, there are problems
1. Like all tablets, the place you draw is offsetted by a plastic surface of some milimetres. This means that you will *never* get so used to it that you will be able to draw on it with the same assuredness that you could with a paper and pencil. In fact this is worse than most tablets because you're looking where you're drawing with the cintiq so this makes you expect it to work like it works when you're drawing on paper. This is why I do the pencils by hand so at least the initial strokes can have some assuredness. How do you combat this effect further? You work very zoomed in (I draw on the cintiq zoomed in and I have a second window open in my regular monitor at 20% zoom, which is right for a 600 dpi a3 comic that I intend to prin in a4).
2. But working very zoomed in has its own share of problems. A lot of high gestural work is lost. Sharp curved lines that we've trained our hands to give us with a pencil for a decade or something aren't as easy to pull off now (not impossible though, but expect a lot of *draws curve* *undos it* *draws again* *undos again* until you get it. And that upsets the 'zen' of drawing in itself). Also working on a detail and then zooming out to see that the detail isn't working is always demoralizing. The way to combat this is that you might have to use a few more automated tools in Manga Studio to get perfect curves, or tapered edges on long strokes. Thankfully, Manga Studio provides these and much more.
3. You have to learn to have a light hand, much lighter than with a pencil. I'm an HB, H2, H3 kinda guy when it comes to pencil work, so I'm used to holding the pencil pretty firmly, pretty much 'carving' the paper. This is a no-go on the cintiq, you have to learn to use the stylus as if you're feathering strokes of watercolor.... even if you're laying down hard as fuck 100% opacity marker trails (which I do much more often than feathering strokes of, slight but not alarmingly so, homosexual watercolor). Perhaps this is my problem only though because I've noticed most pencil art people use much softer B pencils than I do.
4. I'm not sure if it can stand lots of use. I've been taking relatively good care of it but using it pretty much daily for a year, and now on the bottom center of the drawing screen (think of a parallel horisontal part of the screen, centered in the middle lower part of it) has a 'warping'. When I draw a long vertical straight line, it'll be long and straight until it reaches that part of the screen and then it'll go
\ <- ouch!!
this is actually a bigger deal than I make it out to be, but I tend to work zoomed in and draw in the center of the screen (as most rational people do) so I've learned to avoid it. But it's a disappointing aspect of the cintiq. Perhaps it won't happen to you!
All in all, I can't live without it, but I suspect it's more the combo of Cintiq/Manga Studio than just the Cintiq. It's great for what I'm using it, but make an informed decision if you're going to be doing a lot of color work, if you're prepared to sacrifice a bit of gestural fluidity, if you're given to slightly homosexual watercolor feather strokes, so on.