There are plenty of collage apps for iOS. One of the best is Diptic from Peak Systems. It allows you to grab photos from your camera roll and combine them into attractive, framed presentations for sharing.
Now there's something new called Diptic Video, which is US$1.99 from the App Store. You can still use it to create photo collages, but it adds video to the mix. The process is similar. Select some videos that are thematically related, load them into Diptic Video, arrange them how you like and pick a presentation (there are 35 templates to choose from). You can have four windows all with video, or three, or two or one.
Once the videos are loaded, you can play them sequentially, loop the clips so they repeat and stop after the shortest or longest clip. You can also add music that you have on your device. Videos can be played forward and backward. It's a nice idea for creating some novel mementos of the holiday season. When you are done, you can save your creation to the camera roll or upload via email or SMS.
I gave the app a test flight with stills and videos. Using the app is straightforward, but using stills doesn't really go beyond what the original app does. Since the new app is video-capable, how about programming in some moves and little animations with the stills to bring them to life? Or have multiple stills appear in each frame one after the other? The video part of the app is quite good, but when you add music, there is no way to adjust the level, or to balance between the video sounds and the music. It would also be good to allow the raw sound to be turned off. Perhaps that is asking a lot of an app that runs on iOS, but I think it is possible.
I like what Diptic Video does, and it is reasonably priced. I do think the feature set could be more ambitious, and that's where I'd like to see the app go.
Diptic Video requires iOS 7 or later, and runs on any iOS device that can run the latest OS. The app is optimized for the iPhone 5 series.