If you do a lot of moviemaking on your iPhone, you'll probably want something with extended capabilities. There are plenty of apps to help at various price points and feature sets.
Recently-released MoviePro is among the better ones. Although it runs on the iPhone 4 and the iPod touch, you don't get all the features without the more powerful iPhone 4S.
MoviePro offers pause and zoom (4S only), selectable frame rates, aspect ratio, and variable audio and video quality. There is a tap-to-focus feature (additional taps set exposure), auto white balance, a timer and the ability to choose maximum record duration. The zoom feature just magnifies the pixels you already have, but some people will find a use for it.
The app will also allow you to capture a still while you are recording video. Just tap on the onscreen button and the image will be saved to your camera roll. Video can be exported to Dropbox, YouTube, Facebook or sent to your camera roll.
I found the overall video and audio quality to be very good at the best settings. Help is built into the app, which is welcome. I would like to have seen the ability to extract a still from an existing video, rather than being only able to do that live. It's hard to tap the screen and capture a still without moving the phone, potentially affecting your video shoot.
MoviePro is on sale for US$0.99 for a limited time. I think it is a good app for the money, and certainly adds to the standard iPhone video features. If you want more pro features and increased control I'd suggest you look at FiLMic Pro at $3.99.