I bought the No. 50 about a year ago but didn't use it much because the ink flow wasn't that great. Recently I bought the No. 40 and the ink flow was fantastic.
A reader told me that he had bought two Kuretake brush pens and both had ink flow problems. So there could be some quality control issues. But when you get a good one, it's is amazing. More details below.
Difference between Kuretake No. 40 and No. 50
Both use sable for the brush bristles so they mimic traditional brushes in terms of usage.
In terms of design, the No. 50 has a glossy body a two colour gradation of gold and black on its cap and body.
The No. 40 has matte black surface for the cap and body. Both are beautiful and the size is just right.
The clip is gold in colour. When you uncap, you reveal the section with gold trimmings at top and bottom. No. 50 has an extra trimming on the bottom of the cap.
No. 50 comes in a nice wooden box while the No. 40 is housed inside a cardboard box.
The No. 50 is more expensive because of its design and the packaging. Price difference is under $10 on Amazon. Whether it's worth is to pay the extra dollar will depend on which design you like better. The wooden box is nice but not necessary.
When you first uncap the brush pens, you'll notice the tips are wet. To keep the tips sharp, the brush pens were installed with disposable cartridge filled with water. I like it when companies pay attention to the details. Most other brands just ship their brush pens with dry tips.
Below are strokes you can create on smooth paper.
The brush pens are delicate to control and capable of really fine lines to 4mm thick lines. Apply slight pressure and the stroke thickens. They are quite a joy to use. The bristles go back to a sharp point after each use.
On smooth paper, the brush pens are able to produce sharp edges quite easily. However if you draw a bit faster, then you'll start to see the dry brush effect on the edges.
The ink I've used are from the ink cartridges provided. It's not waterproof.
The Kuretake brush pens use Platinum converters and with that you can choose specific inks to use.
Be careful when using pigmented inks in these brush pens. If you really want to use pigmented ink (they are waterproof), check out Platinum Carbon Ink and the Sailor Kiwa-Guro Ink which are both nano pigmented.
I love the ability to be able to cover large areas by using the sides of the brush. On smooth paper, especially if the paper don't absorb the ink quick enough, your wash may dry with some sort of gradation. The sketch of the vintage car above shows the gradation.
While you can use thin strokes for details, you can use thicker strokes for a more stylistic look.
The brush pens produce quite a different look on paper with more texture. There's more tendency to produce the dry brush effect. To produce sharp edges, you have to go really slow, or the better way is to use smoother paper instead.
The dry brush strokes are more textural and create a totally different feel to the artworks.
When you're using it on textured paper, the ink gets spread more evenly.
When the brush pen gets clogged
Be warned that the following procedure may damage your brush pen.
The plastic "ferrule" that's holding the bristles together can be removed. To do so, twist very slightly in clockwise and anti-clockwise manner. You don't want to twist too hard because the bristles will turn with the plastic part. Once it's loose enough, you can just pull it out to expose the bristles. Now you can start the cleaning.
Put the plastic "ferrule" back very carefully so as not to damage the sharp tip of the bristles.
If that does not remove the ink clog, just buy yourself a sable brush tip replacement (shown above).
If your brush is too worn, you should also get the replacement sable tip. Generally speaking, the sable bristles should be able to last quite long with proper care.
Update 2020: This is how the matte brush pen looks after a few years. It looks like humidity and the matte surface don't like each other.
The Kuretake sable brush pens are the best compared to the other brush pens I've used. The caveat is you might need some luck to get one with good ink flow.
It's quite pricey but it's definitely worth the money.
To check out other brush pens, visit https://www.parkablogs.com/content/brush-pens-compared-drawing-purposes
I suggest getting the brush pens on Amazon as they have better prices. I got ripped off buying from some other website that I shall not name.
The links below go directly to the brush pens on Amazon.
Try searching Jackson's Art Supplies too.