It does seem to be a little redundant to kids who learn fast. I would say it's good for introducing totally new concepts. I also wouldn't use it exclusively, but I think it's engaging enough to use as a supplement.

Do they allow you any access for a free trial? DD6 uses grade 2/3 and DD8 uses 4-6. You might assume your children will have exhausted it well before the end of the year subscription, though.