It's not. Atleast not the one I ever wanted.
It's barely any better than the other failed sequels that have come since 1991. I might actually prefer some of the others, as much as I disliked them at the time.
Looking at it objectively, it's not an awful movie. But they built it up so much with the return of Cameron and saying it was a 'return to form' etc that I think the disappointment has hit twofold. It is a very, very average film.
I'm conscious of some 'gender politics' attached to this movie, or how it's perceived. Please understand that I was fully okay with having three female leads. Terminator has been the tale of 'the mother of the future' all along after all. Let's be clear that it was just a fairly poor film, regardless of gender.
There are still only two films as far as I'm concerned - T1 & T2. If you want the real follow up, get hold of copies of the comics 'T2: Cybernetic Dawn' and 'T2: Nuclear Twilight.' (Read them in that order). They wrapped up the story tidily and respectfully. This film achieved neither of those things.