"I have gone back and forth between water brushes and regular brushes, and it looks like the water brushes have won. Water is always the biggest issue when sketching on location, and carrying a water supply is the biggest burden in terms of luggage space and weight. A water brush requires less water to clean since it is more efficient to channel a small amount of water through the brush than to swirl the brush in a cup of water, dirtying all the water. If you do watercolor sketches on location and don't work any bigger than 9 x 12 inches (or F3 size) than you ought to give the waterbrush a try. It could make life easier for you and even change your style."

"The water brush will never render traditional watercolor brushes obsolete, but I think it will eventually become the preferred tool for the majority of watercolor sketch artists, especially those who are just beginning to sketch and paint. And keep in mind, the waterbrushes of today may be considered merely the first generation; improvements may come which could seduce even the die-hard purist."

source: http://www.stutler.cc/other/sketchbook/waterbrush.html