Macworld’s review of Adobe Illustrator CS5 has the following to say about the new Bristle Brush tip feature:
A new Bristle Brush tip mimics natural media painting while remaining vector. Creating a new Bristle Brush lets you begin with either round or flat brushes in one of five styles each—point, blunt, curve, angle, or fan. From there the brush can be completely refined with options for bristle length, density, and thickness, paint opacity, and stiffness. The results of painting with Bristle Brush brushes are astoundingly natural. Illustrator won’t yet replace Corel Painter for lifelike, natural media painting, but then again, Painter doesn’t paint in vector.