I think that Photoshop is actually good for making nice clean line art. The underlying problems are usually: 1) lack of a tablet, 2)lack of experience, 3)methods/technique/settings.
Some programs like GIMP and SAI have smoothers or stabilizers that can help compensate for the deficiencies listed above, but they are not a complete solution. If you decide to address all of the issues then your end result will be better.
Yeah...about SAI...I have a Mac, so I'm sure you know where I'd be running into problems However, I'm planning on getting a Bamboo Splash Tablet, which will apparently have ArtRage 3.5 bundled with it. And from what I'm seeing, ArtRage is supposed to be really good, so I'm a bit more hopeful!
Normally people don't make line work directly on the computer, like you would an anime/manga style, most draw on paper then scan. And if they do do it on the computer, it's because they're very well practiced at it. You can always change your line work if it's not the way you want it too. Most create line work on the computer just as a rough sketch to guide them when painting, and either there are no lines in the finished piece or the lines get added carefully right before finalizing the piece.
If you're looking for recommended software for Macs, then search the topics in the Software forum. There have been many topics about it already. Some choices are Photoshop, Corel Painter, Autodesk Sketchbook, MyPaint, Art Rage, and others.