Vine is ok but you can't say it shines in terms of usability. The major issue is the tap to record. As most of the iOs users are taping to focus, you are basically going against a natural behavior for a camera type app.
I used an app called Dirctr before, it provides you with storyboards that you can fill with videos you record in order to make interesting videos, that are not just long shaky plan of what's around you. http://www.directr.co/ The results are great but I've only used it a couple of times because you have to go back to the story board each time you want to add a new shot. That makes the UX complicated.
I use Vine all the time because the tap to record is the simplest way to make videos composed of several shots. You can even start recording a couple of shots, turn off your iPhone and come back several minutes/hours later to add some more. And iPhones are pretty good at focusing so it doesn't seem to me like it's a big issue.