I went to Blicks today to pick up some supplies and decided to test out some of the Arches rough grain watercolor paper. I had been using the green covered, fine grain paper previously.
Both papers are cold pressed so they both have a certain amount of tooth. The fine grain is noticeable smoother, but there's not a huge difference between the fine grain and the rough grain. I set up outside to do quick light and shadow study and found that the rough tooth really works well for capturing a quick effect of the light and color. I wouldn't recommend using the rough grain if you're a more detailed painter, but this rough paper works very well for me. I also found that it's not as rough as the Winsor & Newton watercolor paper.
Gotta keep painting while the weather's still fairly nice; once it gets colder out, the watercolor freezes while you're trying to paint.