I have found that this function is so awesome. Gone are the days of "penning" each individual line in a piece of artwork. I cannot believe how this speeds up my work process. If you have the means, I highly recommend Illustrator CS5. Here is an example of line art that I did of Tinkerbell.
Original sketch copyright of Jessica Hickman.
jessasketch on deviantart.
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Variation of traditional colors.
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Traditional colors.
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Line art completed in 2 hours using the variable width pen tool.
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Orignal sketch created by
jessasketch