Once processed, you can save the image to your camera roll or share it via the usual iOS share menu options.
Here's the thing: The software works, but you really can't see a significant difference on the iPhone's screen. At best, it looks slightly less grainy, and it does have a softening effect on skin which might be helpful in some cases. For dark photos, you still need to use another app like Snapseed first to adjust white balance and exposure -- Noiseware's exposure slider provides minimal brightening -- and those operations deliver more obvious improvements than noise reduction.
If you plan to upload the photos elsewhere, for use unfiltered and for viewing on larger screens, then it's more worthwhile. Pronounced color noise in shadow areas gets cleaned up nicely and looks better even at small sizes. If you post images to Facebook or other social networks at medium sizes, then it's probably worth keeping Noiseware around for low-light shots. However, if you want to use the images at full size, I suggest you bypass the phone-based corrections and do them on a computer; it's hard to judge on the small screen if you're overdoing it, and on a computer you have more tools.