I bought this veyr soon after it was released on Steam.
It was an impulse buy, however I do not regret it.
My biggest regret is not owning a tablet; Be it a drawing tablet like the Wacom Bamboo, or a Tablet PC.
The only negatives I can think of would be that it can crash from time to time, depending on certain actions. For me, it was quickly hitting undo and creating a new layer directly after. I've got out of that habit, and have yet to have it crash on me.
Two other negatives would be the limited zoom, up to 100%, and the limited layers, up to 8.
Saving your work is REALLY quick. On my computer, it happens in less than a second.
Brushes are extremely expendable. Create and share your own, download some from others, certain brushes will react to velocity and pressure, etc.
One really neat mode is how you can use the color of an image.
Essentially, when doing this, BlackInk is acting as a photo-filter, and only applies the filter wherever you paint.
This can create some really cool photos and effects; Although if you're planning on 'filtering' an entire photo with a specific style of brush, it'll take a while. (Since it's all by hand.)
All in all, this is a great program.
Granted that it only has two tools: The brush and eraser.
It's still really great for doodling around. It would be very ideal for someone with a tablet... Not so much for mouse-only users.
Overall, I highly recommend it to artists.