I used my iPhone to photograph fireworks last night. Slow Shutter Cam, Light Trail mode. I set the shutter to B, started it when I heard the "Whump" and stopped when I thought the painted image looked about right. Real scientific process here.
One of the things I remember from DSLR shots of fireworks is that it's really easy to overdo it. A few bright flashes and the image is a mess. With the iPhone on Light Trail it paints the image onto the screen so it's a bit easier to judge. About a third of the time the camera was pointed wrong (I was close, almost directly under the display), and about a third of the time the brightness of the firework just overwhelmed the sensor. The other third I got what appeared to be a good image, but when I got home I discovered about half were out of focus.
I haven't figured a way top pre focus this App but have written the developer.