It's a very cool concept but it needs to be fleshed out a lot to make it worth the purchase price. I would say that given the lack of diversity within the exercises that it is not a very effective learning tool in its current state.
I liked:
The drill concept seems like a good way to learn quickly
The timer keeps track of your progress
Instructions are concise
I didn't like:
Not very many exercises
Doesn't cover all of Vim
Not enough different tasks within each exercise: typically it is 1-3 tasks that move to different line numbers.
Doesn't keep track of your speed so you can't see if you are improving