Yoho, I think you are right not to underestimate the staying power of movies with geek appeal. Harry Potter will do that for a younger generation. Let the intergenerational battle of the geeks begin!!
AV points correctly that for HP the (extra) $100M question will be whether the film can reach out to the date crowd. I think it can, but, boy, are there a lot of good choices this season.
DanG