Noodlers X-Feather is designed to minimize feathering and bleed-through and accomplishes this by being more viscous than other inks while still not having added pigments or particles that could hamper flow. This allows the ink to more or less sit on top of the paper rather than be soaked into it as quickly as other inks might.
Like most of Noodlers' inks, X-Feather comes in a basic 3 oz. glass bottle. The bottle is on the tall and narrow side so you may have trouble filling your pen as the ink level drops unless you have ink syringes or eye droppers. If you don't want this to be an issue, you could do what I do and buy empty ink bottles to decant the Noodlers' ink into. They aren't very expensive and can prevent the frustration of trying to get a pen deep enough into the bottle.
X-Feather is one of the blackest inks out there and it's because of this that it is often recommended to calligraphers both because the darkness will hide any errors and the viscosity will help it cling to dip pen nibs.
X-Feather is Bulletproof, meaning it is water resistant and can resist fading due to light exposure. I often use it at school because my notes can then be thrown in my backpack next to my leak prone water bottle and I can sleep at night.
Flow has never been an issue with X-Feather for me and like many Noodlers inks, this one is on the wet side.
Because of the high saturation and darkness of this ink, it does have some show-through (not to be confused with Bleedthrough) even on thicker and more expensive papers. If using both sides of the paper is a priority for you, you might not like the way you can see the letters on the other side through the page. This can be worsened with the use of thinner papers like Tomoe River.
On less absorbant, more expensive papers, X-Feather can take minutes to dry. This is most likely due to the fact it is meant to sit on the page rather than soak into it. If you are left-handed and run your hand over your words as you write, this will be a problem.
If you want an ink with low-shading, high saturation, good water resistance, good flow and long dry-time, this is undoubtedly a great choice for you. However, if you frequently use less absorbant, more expensive papers, or you are left-handed, you may not enjoy it.