The app features a unique interface which I found confusing at first, but once I read the tutorial it made a lot more sense.
Addr differs from most reading apps in that you manipulate the text (whether it's adding a note, jumping to a different section, or what have you) in the margins and not the text. Adding a note involves a swipe gesture in the right margin, and you can advance the ebook page by page by swiping the left margin. This takes a little getting used to but once you ascend the learning curve this app is as easy to use as any other.
I played around with it this morning, and while I am not a fan of scrolling vs page turns I did appreciate some of the details like the location column in the right margin. That is displayed as microscopic text rather than the simple location bar used in many reading apps. The text is too small to read but its general shape could be useful in identifying the location you wish to jump to.
You can find the app in ITunes.