I don't even hatethe final form in the miniseries - I just think the problem is you need to convey that there's something behind it that just can't be comprehended. I think with digital filmmaking and effects that might be easier to illustrate. In the book there are sections written from IT's perspective at the ending and there's no real good way to convey that on film.
I don't find the ending disappointing in the book at all.
What does IT say in the book?