The basic functionality of this is fine. It does what it’s supposed to do, and does it better than most other apps you’ll find. The biggest advantage it has is in how all the notes and evaluations you have are attached to the actual position, not the move list. This means it doesn’t matter if you arrive at the position from a different move order, it will connect you to all the notes you previously made about that position.
But there are gotchas:
There’s no flexibility in the layout. Among other things, this means if you want to write a text note about the position, the edit box it opens obscures most of the position you want to write about. Being able to do simple things like resize and/or relocate the board (or edit box) on screen would fix that.
It fails to take real advantage of the cloud. You can’t work with or directly store files in the cloud. You have to pass everything through Dropbox. This adds an extra process that complicates sharing data between phone and tablet, for example. (“Did I upload those changes to Dropbox from my phone? Is the latest version on my tablet or my phone, or in Dropbox?”) just giving it the normal iOS files interface would fix that.
It’s a useful tool, made a little less useful by some less than optimal design decisions. But if you don’t mind having to create a Dropbox account just to transfer data between devices (my biggest issue with it) and you can work effectively with the developer’s choice for screen layout, it’ll help you succeed.