I recently discovered this app, and as you can read below, I like it a lot. But feel free to tell me why a different app is better.
First of all, this app really shines in training you to write and remember the kanji. In this day of word processors, it might seem that for most practical purposes, all you need to do is recognise each kanji. From sad experience, I can say that until you can write the kanji, you haven't really learned them, and you'll be prone to confusing similar ones.
I skimmed through this thread, and didn't see this software (iPhone and iPad two-in-one app optimised for each) mentioned anywhere. Although Kotoba! is a really awesome app, especially considering that it's free, I prefer Kanji LS Touch by far for studying kanji systematically. In practice mode it allows you to paint the kanji with your finger, and you can turn cuing on or off. In cuing mode, it shows a faded version of the kanji in the background (either system font or handwriting font) that you want to paint, and indicates the next stroke and where it starts. You can and should turn that off after a few practices so as to exercise your own memory, but you can always toggle it back on. The kanji set you want to study is highly configureable, as is the way you test yourself. Generally useful predefined kanji sets come with the app (grades 1, 2, 3, 4, 5, 6, secondary school kanji, JLPT N5, N4, N3, N2, N1, all joyo kanji, and all kanji, as well as custom sets that you create and import yourself via a free service such as dropbox.
As mentioned, the flashcard tests you set up are highly configureable, with reading to kanji, kanji to reading, meaning to kanji, kanji to meaning, fill in the missing kanji, etc. with options for either timed multiple choice or your typed-in answer or painted-in kanji. When you paint in the kanji, you end up being the ultimate judge of whether the kanji is right or wrong. To decide, you touch the "solve" button, and the kanji you just painted is displayed alongside the correct kanji (again, toggle-able between system and handwriting font). But one of the refinements that make this an absolutely best-of-breed kanji learning app is the "stroke" button you can push to check your work while you're on this screen. Push it once, and you can see both the correct first stroke and where it starts juxtaposed against your first stroke and where you started it. Pushing this button repeatedly, you can step your way through the entire kanji, making sure you got all the strokes right. If you got the order or direction wrong, you'll notice it, and if you left one out, even a "minor" one, you'll really notice it when the highlighted strokes get out of sync as you step through. And even if you got it right, this serves as one additional reinforcement of the correct stroke order. Again, it is up to you how stringently you want to grade yourself. If I'm off with the proportions, but get the stroke order right, I score it as "right" even though an elementary teacher might mark it "wrong." But if stroke order doesn't matter to you, you're free to ignore even that.
Also, you can set the language interface and meanings to display in English, French or German.
Also, like many flashcard kanji learning systems, you can turn on the Leitner system, which basically keeps track of which kanji you're having more trouble with, so that you can study them more.
My only quibble with this app is that the choice of kanji-related vocabulary is occasionally curious in containing seldom-used words.
Many will be disinclined to purchase a $12 app when a very good app like Kotoba! is already available for free. A miser by nature myself, I can identify with this attitude, but it is a false economy. How much is your time worth? Kanji are absolutely essential in advancing your Japanese language skills, but they are a high mountain to climb. You should give yourself every advantage for mastering them efficiently!