I shoot photos with an SLR and my iPhone - its funny how the roles have reversed over the last couple of years where I use my iPhone more then my SLR.
But I have always wanted some amount of control on my iPhone. I have tried numerous camera apps in the past that seem to give you some kind of manual control, but nothing like an SLR. Enter iOS 8 and Apple has now opened up a lot more on the camera that allows developers to access camera settings like never before.
Manual for iPhone is one of the first app to do so and it allows you to control your ISO, white balance, focus, shutter speed and so much more.
When I am shooting on my SLR, I am always checking the histogram on my pictures; just to make sure my exposure is correct. This is the best feature of Manual, it shows me a live histogram on the app while shooting.
At $1.99 its a steal.
Note: If you don't shoot with a SLR; avoid this app.