I've used a Wacom stylus for digital signatures for years, and find it quite useful for that aspect. Your mileage may vary. As for drawing straight lines, that not only depends on the sensitivity of the pen, but also the steadiness of your own hand just the same as a traditional media pen. Often times, I'll use the steady-stroke option in sculpting packages with a mouse more often than a stylus because my own hand isn't very steady.

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