I have not used the Neckline Slimmer (and don't personally know anyone that has) so I really can't give you any real reviews.
But I have seen the commercials and ads (in fact there's one showing on this site right now LOL!) and here's what I think - strengthening muscles along the neck and jawline is probably not a bad idea. Will that lift the skin? I don't know. In theory it makes sense, since flabby legs and butts perk up when they're toned. But whether it helps under the jaw too, I have no idea.
If it were cheap enough and I saw it on the shelf at Target or the drugstore, I might pick one up just to try it out. That way I could return it after a few weeks if it was a scam
If you have a really awful jawline (or should I say, if you don't have a jawline!) then I don't know how anything other than a necklift surgery will correct it.
I know this may not be your idea of a Neckline Slimmer review, but hopefully I've at least helped you a little