This ink is not as water or smudge resistant as the pictures in the description indicate.
Presumably this ink, being carbon pigment based, is made for artistic applications, if you intend to use it for art or illustration it may however be disappointing. Pens like microns, pitt and staedtler use pigment ink because it is waterproof, permanent and can stand up to erasing and washes. The sailor pigment ink, though it handles well in fountain pens and is very beautifull, bleeds considerably, even when fully dry, when gone over with a wash and it smudges and smears pigment when rubbed with an eraser. These two qualities make it a bad choice for pen and ink illustration, though still quite nice for sketching and looser, free hand drawing where erasing is not required.
You can see clearly in the pictures above that the stroke that is washed over is a very thin layer, almost grey and therefore has less pigment to bleed. When applied at full opacity it bleeds considerably more and can even be reconstituted days later, which is fun if you want to play around with grey washes. This is not something that would be important unless you are an artist and I do not think jetpens is trying to misinform anyone.
Despite these limitations, this ink works beautifully in flex nib fountain pens, is great for figure drawing and sketching or calligraphy. It dries to a very nice satin finish which accentuates the texture of papers. I use this ink in an antique flex nib pen with a water brush for black and grey portraits and love it. If you are looking for a good ink to use with watercolour, this is not it, before spending so much on ink you should know exactly what you are getting.

An outstanding ink. Works perfectly in all of my pens, from a broad-nibbed Montblanc Diplomat to an extra-fine-nibbed Pilot. Dries quickly, and is permanent and truly water-resistant on good paper. Not quite as intense a black as Herbin's Perle, or Aurora; but is deep enough, in a midnight charcoal sort of way. This ink is far more refined and less prone to smearing than Noodler's so-called "bulletproof" black, which looks harsh and vulgar by comparison. The only thing wrong with Sailor Nano Ultra Black is that it is so bloody expensive. Still, that won't prevent me from buying it again, as it is by far the best permanent black ink I have ever used.

This is the best black ink I've ever used. Beats Mont Blanc, Noodler's, Parker, Lamy.
Extremely black. Smooth. Doesn't bleed. Doesn't clog. Works well with all papers and pens I've tried.
This ink makes me curious about the science of how ink is designed. Whatever they did, they designed one great ink.

Ordered the ink despite it being a little more expensive than the other inks. Filled a fine nib Sailor candy fountain pen with it, and the ink has a nice expressive quality to the fine lines. It may not be as dark as Noodler's, but more water resistant for watercolor washes. Overall I would reccomend the ink.

bought this as a random purchase, not disappointing at all! goes on smooth with my sailor Regulus fine nib. drys fast, decently waterproof, and really black! works very well on college lined school paper, no bleeding and does not show thru the page. filling system is really cool, bottle is pleasing to look at. I have been using it for about a month now and have not had any issues with cloging or nib creep.