The app aims to be a kind of one-stop app and does give the user features not available in the built-in camera app. One of the nicest is a timer function. You can even set up the camera, and say 'cheese,' or anything else, to get a picture taken when you're not at the camera. Apple should have built this in but didn't.
Another nice plus are camera guides -- superimposed grid lines to help you frame and level your image. Other niceties include a review button to take a look at what you photographed, and a 'big button' that allows you to tap the screen anywhere to take a picture, and not have to find the small icon.
Camera Genius will also help you get a steady picture. If the feature is turned on, the app uses the motion sensor built into the iPhone to take the picture when the camera is steady. It's not a built-in stabilizer, but it will get you a better picture while it waits for your hand to stop shaking. Note: This feature is not of much use if your are trying to get a quick action shot.
There are a couple of things I don't like. The 'zoom' feature really just crops your photo. You are not zooming in, you are just enlarging your image, noise and all. I also noticed that if you are listening to your music collection on the iPhone and bring up Camera Genius, the music just stops. The developer told me he didn't know about that bug, and says it will get cleaned up quickly.
I also noticed that pictures you take through the app have no geotag data. That's going to be a big problem for a lot of users. Jeff McMorris at CodeGoo, which has produced the app, says geotag data should work in the next update.
I'd also like the settings screen to indicate which features are on and off, perhaps with a little check-mark or color cue. If you just look at the screen, you have no idea how things are set.
One of the things I really like in the app is the little camera manual that is built in. It's like a short, basic course on photography, with brief sections on things like lighting and composition. Serious photo hobbyists won't need it, but for people just getting started it is welcome information.
The app is on sale for US$0.99 which is a nice price for such a versatile addition to the iPhone. If the geotag issue is fixed, I think many people would see Camera Genius as a viable replacement for the built in camera app.
Here are some annotated pictures of Camera Genius in use: