I had a hard time figuring it out- what the task was, and how it worked. For example, it stops when you make a mistake and requires a new start, where all other nbacks I've tried keep on going for a set number of trials- make a mistake and plow on. It's really fast, too fast, and requires instant counts of objects of up to 9 at once. I can't complete the count in the time allotted, so I'm failing not due to memory but simply never getting the answer in the first place.
I'll probably try it again, but I'm off to a bad start and not liking it.