We have had this box for a while now; we first got it when my son was 6 and, now my daughter is also 6, it still gets as much use.
It has been one of the best toys to bridge the gender gap; both children find it hilarious. The ideas sheet is very simple to follow, but both children love to use their imagination and have come up with puppets for a show (performed from behind the sofa), mummy and baby creatures, flying monsters, etc, etc. all of which are funny and charismatic.
My son is slightly dyspraxic and has trouble with some fiddly construction toys, but this has never been a problem and he always enjoys playing with it.
The pieces are also big and chunky, so don't tend to disappear up your vacuum cleaner (unlike K'nex grown up brother). There are plenty of pieces in this box, so two children can play well together without too many arguments.