The app works great and allows you to record and control your recordings with many options and output formats. Its working great for me (iPhone 6, iOS 10.x). Ive used it with the iPhones built in mic as well as a wired Lab and a Röde Video Mic with very good results. It allows you to vary your gains and boost, apply filters to minimize hiss and even shape the sound a bit, like making it warmer or brighter sounding.
Pros: it gives you the basics of a professional field recorder without having to carry one. The app capabilities are very good.
Cons: the app interface is... ummmm, challenged. It is poorly designed from a usability standpoint, but works. Its just quirky and a bit dated, but it is easy enough to use. Example, on the record screen it shows you a live levels gauge, this is great so you can avoid clipping and set your gains appropriately. The issue, to set those levels you go to another screen and the live levels disappear. Really Röde? That type of thing.
There are some settings that can make your internal mic not function, but going back into the app and changing those brought everything back to normal - but I could see this confusing some people. Easy to correct though.
Overall, an excellent app and field recorder, great features albeit with a poorly designed user interface. The results I e achieved on voice overs and narrations have been excellent, as well as on the fly field recording. Highly recommended.