Yeah, I agree with keii4ii -- there's a big difference between "Aha! I knew it! :D" and "psh, saw THAT coming" even though both are responses to guessing the outcome. The first one isn't bad at all! But the second one is something you want to avoid.
I think this is a good thing to keep in mind even if someone hasn't predicted the ending yet -- that it's a big risk to just rely on folks' getting their minds blown by the surprise of the ending; it's a good idea to focus on making that ending satisfying, too.
Like, if you've figured out the main characters are going to get together, you'll still enjoy watching it happen -- it's not "predictable" when a story does the thing you've been looking forward to. If you've figured out that jigsaw is manipulating everyone to..... have to saw off their own arms or whatever (I'm not 100% clear on what jigsaw does) then there can still be tension and dread in watching that inevitable doom approach.