Black Ink is an... Intriguing tool.
I got this from a friend as main drawing software, since I can't afford the really shiny stuff like Photoshop and don't like getting out the old bottle of rum. Though it's really powerful, there are also some annoying drawbacks, mostly regarding the beta stage of the product.
First, the biggest and most annoying drawback: Bugs. There are times when you boot it up and try to draw with your tablet, it's completely out of calibration - something that isn't supposed to happen. The computer certainly detects the tip to be where it should be, but the program just paints a few mm to a few cm off to the left (or to the right, depending on its mood on that particular day). Besides that, Black Ink has the nasty habit of crashing from time to time, but as long as you save often that shouldn't be too much of an issue.
But, unfortunately, the topic of saving brings me to the most annoying beta-related downside: Corrupt savefiles. It happens, is really frustrating and I hope it gets solved soon. For now the way around it is simply to keep a lot of backups, but really, this needs fixing.
The last drawback isn't all that bad, but just a bit annoying: You can't zoom further than 100%. They're working on this, though, and I'm sure it'll get resolved soon.
Now, enough with the negatives! Positives!
Let me start this section off with what I am sure is the biggest selling point: The brush editor. You can create or download countless brushes, taking parameters as speed, pressure and direction and using those definitions to change your brush or add something or whatnot. I don't know much about it, but it's still pretty damn cool and, if you're looking for this, it's a selling point.
And besides that this is just a really solid tool. It's responsive, there are nice tools to add saturation to colour instead of having to change the hue up constantly to shade, pretty brushes... Fun!
All in all, though the beta stage certainly brings its drawbacks, this is a solid tool. If you're interested so far, buy it!