Black Ink has become my favorite drawing/painting application and I do recommend it, but you should be aware of some of its limitations before you decide if it's right for you.
The first limitation is that it requires a powerful graphics card to operate. Don't expect to run this on your laptop, much less your Surface tablet. The lag will make it unusable and some brushes won't even render. If you're looking for something to take with you so you can paint under a tree by the lake, this isn't the program for you, unless toting a massive rig on a picnic is your thing.
The second limitation is that the selection of brushes is very limited and not entirely satisfactory. While the software is certainly able to handle them, it contains no traditional brush/knife/spatula effects, giving you instead a number of flashy, but not very useful "computery" ones. Bleank's forums include a "brush exchange" where you can get another 20 or so brushes, but half of those are just experiments from people who are learning how to define brushes and most of the rest are interesting, but exaggerated effects of limited applicability. You might find about half a dozen actually useful brushes, including an impressive flame brush, a decent graphite pencil, and a nifty comic book hash shadow brush. The forums also contain a few tutorials on how to make your own brushes. How effective these will be depends as much on your own skills and background as on the tutorial itself. Making a custom brush is as much a technical endeavor as it is an artistic one.
The third limitation is that there are no geometric tools whatsoever: no lines, circles, arcs, rectangles, spline curves, etc. Their addition would make the program enormously more useful for certain types of illustrations, especially if they were given parametrizable "organic" effects (like random variations) in addition to being rendered with the current brush. Given how easy it is to do this (yes, I am a software developer and yes, I have written drafting programs in the past), it's difficult to justify their absence.
Also, keep in mind that you will need a drawing tablet and a pressure-sensitive stylus to get any decent results. Mice and track pads just won't cut it. This is true of all paint programs, but not everyone will think about it until they've tried and struggled with an inadequate input tool. I'm using it with a medium-sized Wacom Intuos Pro and it works great. You might consider upgrading to a stylus with tilt sensors because some brushes are tilt-sensitive and you'll get even better, richer results. Because Black Ink works best on large drawings, I recommend a large writing surface as well, though that's largely a matter of personal taste; I prefer to have room to move my arm rather than just my wrist.
But even with these limitations, what it does, it does very well. I am particularly in love with its layering feature. In addition to letting you stack backgrounds, foregrounds, and various intermediate slides, it's also great for experimenting on one layer without damaging parts that you want to keep. Which brings up the fact that Black Ink's undo feature works very well. This seems like a trivial observation, but I've used programs that make you want to throw the computer out the window either because they undo too much at once or they can't undo enough.
So I certainly recommend this program to anyone who wants a pleasant sketching/drawing/painting program, but I also recommend to Bleank to add more conventional brushes (and why not some more unconventional ones, too?) and a small panoply of geometric tools to make this a more complete and satisfying product.