And I bought it even though I use AnkiDroid on my Samsung Note 3.

Why? Because it is the only way to support Anki's sole developer. He cannot take donations due to Japanese laws. AnkiDroid also does not take donations, though I do intend to start contributing code to that project.

If you really enjoy Anki, consider buying the iPhone app even if you don't have an iPhone.

I was taking a Japanese course couple of years ago. The course was difficult and if you did not pass mid-term and end-of-term exams you had to repeat the term which costed a lot of money. Fearing that I could fail I decided to invest in Anki. Needless to say it was the best spent $25 ever.

$25 is surely expensive for a simple app, especially if you are a student without a steady income. But if anyone wonders whether Anki is worth this much money - try thinking of the potential benefits.

If the Anki iPhone app were the only way there was to do spaced repetition, I wouldn't hesitate to pay $25,000 for it. $25 is a pittance for the work Damien Elmes puts in, not to mention his efforts in maintaining the sync server so you can easily stop and resume your sessions on any device.

While it won't work offline, you can use the web version on iPhone (https://ankiweb.net/about).

There’s now a free app that seems to do the same thing.