This review is by Kalina Wilson, who can also be found at Geminica.com.
A frequent complaint for brush pen users is that the ink doesn't flow freely enough, resulting in a dry brush effect where you really didn't want one. You'll get that effect if you use toothy paper or if you go fast enough, no matter what brush is used - see the right side of the above drawing - but the greatest strength of the Pilot Pocket Brush -Hard is that it really wants to lay down an easy lush sloppy black line, packed with ink.
You can see here that by comparison, the Pilot brush makes the lines from the Pentel Pocket and the Kuretake No. 8 Hair Brush look grey. They were stocked with good black ink, so this difference is probably entirely due to the difference in ink flow.
Unfortunately, while the Pilot does lay down a nice black line, it is also a very thick line. I found it a struggle to control this pen. Here are some loose comics I made using the Pilot hard brush which is maybe a better direction for this particular tool.
With a wetter application comes more smudging. I recently spent a few minutes sketching at Portland's Pioneer Courthouse Square and couldn't avoid getting my hand in the ink (smudges are visible on the main figure's backpack). This drawing also demonstrates that the ink is not waterproof. It holds up okay if you touch it with some watercolors on a just barely moist brush, but if too much water gets into the paper it bleeds in an ugly, feathery way (see around the bird).
In the end, I feel fond of the Pilot Pocket Brush for its lushness but it isn't really a good match to my needs. I recommend this brush pen if you want something very affordable, prefer a strong ink flow, intend to draw fairly loosely, and aren't planning to add water colors. It could serve that purpose quite well. Otherwise - your perfect brush pen is elsewhere.
Confession: This review was originally going to cover both the hard and soft brushes, but I lost the soft brush while out sketching! I was surprised to find that despite having totally different tip shapes, they produced a similar line. The soft brush might make it easier to avoid the dry brush effect simply because the tip is so much larger, but I preferred the hard brush because it was easier to predict its movement.