I think the kiddos like Snapchat partially because it's not FB, or Instagram. I think especially you'll lose them once they have to start thinking about who's seeing what.
Nevertheless, I imagine non-tween Instagram users will enjoy this addition. Personally, I like the idea of ephemeral photos. For some reason I have some stupid mental block to sending pics, even though I know its easy to delete them from my album.
And in this case, I think all the additional features that comes with IG Stories is actually very cohesive with the rest of the IG features. All the new stickers, overlays, filters, and I'm sure they'll be adding the same animated filters are all very natural extensions of the IG user experience.
So many people use Snapchat as a photo/filter editor to then share on IG/Twitter/FB, now they can do that very same experience directly in IG and share it with their existing fans.
And maybe an overlooked aspect is IG's better UX vs snapchat's bizarre stubbornness to maintain their oddly user antagonistic UI.
But like I said, for everybody else, I think it will go over well.
In other words, I think absolutely no tweens are going to abandon Snapchat for this. And that's perfectly fine so long as they're not expecting them to.
Working on this project at Facebook must have felt like being Goliath and trying to kill David.
I hope Snapchat crushes this.