I finished the first Dragonbox in one day and had to buy the second one, which I came close to finishing that day as well!
Despite the fact that it was a total of $16, I think it will be well worth it for both of my kids. They slowed down on it, mostly because it's on my iPad, but they'll both get through it eventually and we can always drag it back up when it becomes necessary for various bits of learning. A nice refresher course, as it were.
I suppose I'll probably have to buy it again for their tablets that they are getting for Christmas. That's when it starts getting expensive!