It does not make lineart any faster. Seriously. I have one, and I end up using the laptop mouse in Illustrator when I need to do clean lineart (admittedly, this is not often.) It only makes it easier to tell what you're doing over a normal tablet.
Pressure sensitivity doesn't mean cleaner lines. If you have a jittery hand, it means it'll pick up that crap too. Also it tends to screw up its calibration near the edge of the screen.
If you're only worried about lineart, the Cintiqs are not worthwhile for you.