Odds are you are satisfied with the photos you snap with a late-model iPhone. My iPhone 4S, for instance, captures higher-resolution photos (8 megapixels) than my old 6-megapixel
The app's layout has you hold your iPhone in landscape mode, with the rear-facing camera in the upper-left corner (645 Pro does not bother with the lower-resolution front-facing camera). Controls are packed in along either side of the live preview pane. A row along the bottom edge mimics the LCD of a SLR and shows your current settings, including exposure and shutter speed, meter mode, flash, development status (JPEG quality and whether you'd also like to save a TIFF file), and a histogram. These are all of your controls, available right on the screen. No complex menu system to drill down and navigate, although each button has a secondary function you can access by tapping and holding.
I could go through and touch on what each control does, but I will turn it over to 645 Pro's clear and thorough user guide (accessible by tapping and holding on the play button in the lower-left corner). Here are three screenshots that show what the left and right controls do and the various settings the bottom panel displays:
The controls mimic the feel of using an SLR. When setting up a shot, you can use the AF-L and AE-L buttons below the shutter-release button to lock the focus or the exposure of a shot. You can also tap and hold the shutter-release button to lock focus or exposure, similar to pressing the shutter-release button halfway down on an SLR. The app features both multizone and spot metering, night mode (shutter speed up to 1 second), and two grid modes to help you compose your shot. There are also five backs (square to a panoramic 6x17) and seven film stocks (five color, three black-and-white) from which to choose. Other than color vs. black-and-white, the differences in the various film stocks is subtle; Instagram filters these are not.
Given the audience for this app, it's likely most users will thumb their nose at using a digital zoom, but should you want to zoom in on your subject, you can pinch the screen to engage the digital zoom (up to 2x).
To select the output options, tap and hold the Grid button. There are four options: high or lossless JPEG, each with or without also saving a TIFF file. The JPEGs are accessible right from your iPhone's camera roll, but you'll need to sync your iPhone and grab the TIFFs via iTunes. In iTunes, click on your iPhone from the left panel, click Apps from the top menu, and then scroll down to the File Sharing section, where you'll see the 645 Pro app listed and your TIFF files to the right. Of the handful of TIFFs I looked at, they ranged in size from 6.8MB to over 13.9MB.
You can also view JPEGs from within the app by tapping the play button in the lower-left corner. You can't edit or delete JPEGs from within the app, but you can e-mail them or share them on Flickr, Facebook, and Twitter.