Tombow Dual Brush Pens are actually water-based markers with a brush and fine tip.
The brush tip is the sponge type unlike the ones from the Pentel Pocket Brush Pen or the Pentel Color Brush Pen. The brush tip is rather stiff, although it is not very different from other brands.
Compared to the Sakura Koi Brush Pens, the Tombow ones are much longer. So the ink capacity is probably higher. The prices of the two brands are very similar.
Here are some swatch comparison. The top row are from the Tombow brush pens and the bottom from Sakura. Notice that the mid-tone grays from the Tombow are quite similar. The labels on the pen body does not reflect accurately the difference in contrast and tones.
There's one colour from the Tombow set that's quite out of place. It's like a cool lavender version of gray. The other grays look neutral tone to me.
The Sakura Koi grays are all over the place with warm and cool shades. Even shades in the warm and cool family sometimes look a bit out of place.
The brush tip allows you to achieve thin and thick lines depending on the pressure you apply. However, the variation of thickness is not as wide compared to the Pentel Color Brush Pens (shown on the right). The Tombow dries quickly like normal markers but the Pentel Color Brush Pens requires a bit more time to dry. The other difference is, the Pentel Color Brush Pen can deposit more ink when you lift so it requires more care to create an even wash, and to prevent back-runs.
The water-based markers blend quite well. Although if you use it on lousy paper, the paper fiber might come out with the second layer. It's better to use these brush pens on durable paper, e.g. watercolour paper.
They are not water-soluble when dry, so you can use a waterbrush to dissolve the ink and blend with water.
Unlike alcohol markers, these do not bleed to the opposite page. They also do not smell. The ink is acid-free and toxic-free.
And since they are water and dye-based inks, they are not archival and will fade when expose to light in the long term. If you use them in sketchbooks, it's probably not a big deal. For casual use, the brush pens work well.
There are no refills so if you want to refill them you have mix some dye-based fountain pen inks on your own.
Here's a sketch coloured with the brush pens. They are easy to use and great for tonal studies. I did not use the lavender-cool gray.
You can find the Tombow Brush Pens on Artzo: https://www.artzo.in/product-page/tombow-dual-brush-pens-set-of-10-grey-scale
Tombow offers a total of 96 colours in the range. Artzo sells them in different sets.
There's also the jumbo all-in-one set with a stand.
Shop for set of 96: https://www.artzo.in/product-page/tombow-dual-