I think this will be big, specifically the video aspect. Removes the biggest problems with video chat (Skype, hangouts etc.) feeling 'weird'. My immediate thoughts:

A) Don't feel like you're getting roped into a long conversation (you have to hold down on the screen, so of course it will be short!)

B) Doesn't feel like you are bothering people by calling them without knowing if they are free to videochat (they are clearly using snapchat at that moment so they're probably not too busy)

C) No need to have something specific to talk about; can just say hi quickly and see what your friends are up to.

D) As a bonus, if you are messaging with someone in the text interface and get to a point in the conversation where you need to communicate something complex, just use the video chat feature for 20 secs. and then go back to the more async g-chat style text messaging.