There are a lot of brush pens out there that look similar to this design: a bristle brush tip, and huge barrel, which doubles as a replaceable ink cartridge. It's a bit of an odd design compared to the more fountain pen-like brush pens, in that you have to squeeze it to push ink into the feed area. These pens aren't nearly as nice to look at, but it's currently my favorite brush pen. This is a utilitarian, function over form brush pen, and it's one of the cheapest of the refillable bristle brush pens by far (usually around 6$ on Amazon or $8.25 on JetPens). If you don't have one of these pens, I suggest you grab one.
First things first: You gotta take out that red ring. That's what prevents the brush pen cartridge from being punctured until you purchase it. And to unscrew the barrel, its righty loosey, lefty tighty. I have destroyed a brush pen before trying to force it the other way. You're welcome. Also, when out buying one of these pens, note the gray barrel color. That's what denotes the pigment ink in this line. Pentel makes a near identical brush pen, the Pentel Standard Brush Pen, but the ink is not waterproof at all. It's cool for what it is, but I see no reason to get that one over this one.
The differences between the extra fine and medium are minimal, just in the brush tip is much wider, and the caps are different colors: the Extra Fine is clear, the Medium is black. I find that the Medium seems to stay wetter longer, but can still usually get the same tight hair lines normally. I do prefer the extra fine normally though, mostly because it feels the most like a #2 Brush in size, which is what I always used with regular sable brushes for inking.
I talked a little bit about the design earlier, and that's one of the best parts about it: you have more control over your ink flow. Most brush pens start off at a good flow when you put in a new cartridge, then they start bleeding ink like a stuck pig after you've used up a quarter of your cartridge. The first time I ever saw anyone mention this phenomenon was Russ Stutler, which he attributed to air pressure behind the ink in the ink cartridge. At that point you have to dab the ink off a brush pen over and over to get it to where you can get hairlines for a few strokes and repeat that process throughout the whole drawing. Annoying. That is much less of a problem for me with this pen.
Depending on how hard you squeeze it, it can get extremely wet, and possibly even bubble out of the the breather hole in the feed and near where the ink cartridge screws into the brush tip, so be careful with that. A little trick I do sometimes is that I will actually just dip the tip of the brush in a little water and refresh the tip of the brush pen, so I can get a wetter line again without filling the ink feed up too much. I squeeze this pen rarely, but you may have to squeeze it more if you like a juicier pen. I have recently become a huge fan of smoother tooth papers, and this pen is great for them. The rougher you go on the paper tooth, the more of a dry brush look you're going to get.
The ink in this brush pen is my favorite brush pen ink. I'd buy it by the bottle if I could. It seems to do better than any other brush pen ink in terms of laying down a smooth, non-feathering line than I have seen from the Pentel Pocket Brush Pen refills (which is usually pretty bad about feathering, to be honest), Kuretake, Platinum, or most fountain pen inks. Really waterproof, and lays down nice and black too. And refills are pretty cheap for the amount of ink in them, normally $2.50 each.
The Pentel Pigment Ink Brush Pen is just fantastic. I got one of these a few years ago and after playing with brush pens ten times the price, it's the one I use the most today. This pen gets a coveted 5 our of 5 Zaffino Crazy Face Franks. Next time you're getting some supplies, pick one or both of these bad boys up.