I somehow only discovered Puzzazz a year or so ago, but since then it's been steadily feeding my cryptic crosswords addiction, and will do so until I run through the rest of its collection (an alarmingly close prospect). I dip occasionally into some of the many other kinds of puzzles it supports, typically regular crosswords and cryptograms, and I'll probably branch out more once I run out of the cryptics. Fortunately, I can also import cryptics from elsewhere on the web, although the tools to do so are clumsy.
Speaking of which, tools and controls in general are the iffiest part of this app. I wouldn't go so far as to call the user interface unusable, but it is uncomfortable. Buttons are laid out in awkward formations. It's difficult to see whether I'm writing in "pen" or "pencil," and I almost never manage to hit the control to switch between them on the first try. The handwriting recognition gets it wrong just a little bit too often to leave it turned on. And so forth. A small number of improvements could make the app vastly easier to use, but it's clear they're not going to happen at this stage.
And yet that's a relatively minor point, because the app is certainly usable, and more importantly, it can handle a remarkable range of puzzles, including a surprising amount of support for the unexpected curve balls puzzle builders like to throw at you. Its puzzle collections are easy to purchase and reasonably priced. (Why, though, why, why can it sync your purchases to other devices, but not your puzzle progress?) All in all, a bit of a pain in the neck to use but makes up for it by being terrific where it counts the most.