I bought this after seeing it in the "Whats Hot" list. Ive been wanting a knot tying app and the references to a book and decent reviews made it sound like a good deal for $0.99. After checking it out, its probably worth what I paid but not more.
First, the "warning" about climbing is annoying every time you start the app.
The diagrams are just simple drawings in a single color, but on the plus side, there is a text description that sorta kinda explains the next step. But It uses knot lingo (like "standing end") that you have to figure out.
It has a useful set of knots but isnt a "reference" on a bunch of knots.
After being intrigued by this app, I did a search for "knot" on the App Store and found a bunch. I eventually also bought Knot Guide (by Winkpass I believe). It is only $0.99 more and not only does it have more knots, but actual photographs augmented with directional arrows, a Favorite Knots feature for quick access, a knot lingo page and the ability to suppress the startup/warning page. You also dont have to rotate your phone. For me, I think I like it better, if for no other reason than it has a few more knots to try.