I bought this and the Kuretake small water brush to see which one had the finer line quality, and I think they're fairly equal. However, I would buy this one again because I can get a better variety of line thickness, and it's easier to refill.

I'm a watercolorist and I love the Pentel Aquash water brushes. I've been able to buy them in sets of 3 at a great price online.
I use the Large brush for Plein Air painting along with a good size 12 synthetic and my go-to Kolinsky hair brush. The 3 brushes in my travel Watercolor kit suit my needs.
The water brush is really convenient in that it holds water in the barrel making cleaning between colors quick and easy. The brush head comes to a nice point and snaps back nicely.

For beginners I would recommend getting a couple traditional brushes to learn with, along with this brush. They handle a lot differently than traditional brushes and I think it's easier to get a feel for how both the water brushes and traditional brush styles behave. Learning on just water brushes can be challenging and misleading.

I don't use this small brush for Watercolor as it's a very fine point and is only useful for fine detail. For fine detail I'm going to lay that in at home anyway and don't really need a water brush for that.
I've seen some people try this size first and I really think that's a mistake. It doesn't give the beginner a fair feel for what water brushes can offer since it's so small.
I would urge a watercolor beginner or waterbrush beginner to get the medium or large size since they can offer fine details with the point of the brush, but also give a little more coverage on a smaller scale painting.

The fine tip offered here is still a brush I have multiples of, but I use them for ink instead. Most pigment ink brush pens have larger brush head sizes. This fine Pentel brush allows me to get a much finer line than some of my larger ink brush pens that are sold pre-filled.
I have several in this size for non-pigment inks and several for waterproof pigment ink.
I fill my main brush pen with Speedball Super Black using a pippette or blunt syringe and it works great.
I haven't had any issues with clogging or flow at all. It takes the brush a minute to become properly saturated with ink so you'll want to sketch on paper with it a bit after saturating the brush tip to be sure it's ready to go. Once it is, you will have no problem with skipping. I use this to lay in fat 1/4" lines by swiping with the side of the brush and also use the fine point to get a fine line or add detail. I'm very happy with this brush and it has all but replaced my Pentel Color brush that's pre-filled with waterproof black pigment ink.

My FAVORITE waterbrush! It has excellent spring and the odd shaped barrel both holds more water, and makes it easy to have a secure grip that doesn't accidentally squeeze the vessel(?) and release more water.

I personally have one for watercolor paintings, and the rest I use with Diamine ink (also purchased from JetPens) and I made DIY ink brushes!

They hold plenty of water and the size is compact for travel, but the brush is way too soft and flimsy to be useful for me. The springiness I want just isn't there. Depending on your style, these might be great for you, but they did not meet my needs. I still kept them as backup, since they're relatively inexpensive and I enjoy having tools around that are different than what I'm used to. I'm pretty sure I wouldn't repurchase, though.

Lovely to draw with.

I prefer filling these with Diamine Ink over buying brush pens with cartridges because cartridges feel sort of wasteful to me compared to buying a bottle of ink. They can also be filled with Sumi ink, but that clogs much faster.

The bristles are very durable and I've not wrecked any of them yet.

I bought this to do try out basic watercolor. I'm a total novice at art.

It works quite well! Holds a decent amount of water. My girlfriend, who's a professional watercolor illustrator, had never seen those before, and liked it. She didn't fall in love with it, so maybe it's better for beginners/people who haven't been using standard brushes for decades.

I'm happy with it though - it does its job, the construction is nice, it's easy to carry around in a bag.

This is just about the coolest thing since sliced bread, no joke. You never have to worry about dirty water and it uses so little as opposed to traditional brushes. I wouldn't say this should be your only watercolor brush, but this is perfect not just for traveling but painting at home as well. Works best I think with watercolor pans.

This is a great entry-level waterbrush pen. I'm using it for brush lettering and filling it with fountain pen ink rather than water, and it works great for this purpose. The flow is fantastic--not to wet, not too dry. The tip is indeed fine when you want it to be, but it can take some abuse and become very broad as well, then bounce right back to fine. The only slightly annoying thing is that the shape of the pen causes ink drops to collect at the top and sides of the pen rather than sliding down to the tip. But it's not a big deal unless you're obsessed with using every last drop of ink, and it certainly wouldn't matter if you're actually using it as a waterbrush pen.

the best travel brush there is. if it is used very very frequently, you do need to replace after maybe a year and half. i love the brush tip. and "Fine" is not a joke! tiniest tip around.

A nice, decent starter brush for travel watercolor sketches. This has a good point, and with enough control, you can get some nice, fine lines for detail work in your watercolor sketches. I know it's helped me lay in some smaller details when I needed it.

One thing to note on this brush: water flows through to the bristles quite well—sometimes a little too well. At times, I feel like I dispense more water than I need, making it so that any paint I pick up becomes diluted quickly. A remedy for this would be dabbing the brush with a cloth or paper towel and holding the brush at an angle to mitigate the water flow.

Other than that, for what it's worth, it's a quality water brush that has a decent water reservoir and good brush springiness, allowing for all sorts of detailed line variation in any watercolor sketch. Definitely worth a try!