It depends on what you want to do with it.
I'm a heavy UX Write user, it's the app I chose to write my book in. The developer sends out updates regularly and responds promptly to queries and suggestions.
I like the way it allows me to structure my text. For academic or complex documents it's a good choice. Headings, subheadings, an automatic contents page, etc. make the task of structuring a document very simple. The interface is not cluttered. It handles images and tables etc. very well. I prefer it to Pages to get jobs done. I also prefer it to Textilus which is similar yet not so user friendly. I use UX Write on an iPad and an iPad Mini very comfortably. It is certainly a very clear, bright writing environment.
However, certain features are still in the pipeline. For example, you can't currently search for a word in your text. You can't move documents between folders. The app can handle docx but not doc. You can't predict page endings or beginnings as you can in Pages because it's one continuous ream of screen, so when you save it as a PDF you're not quite sure what the document will look like. (Textilus has gotten around that recently, but it lacks in areas where UX Write excels.)
I use UX Write because it suits my purpose and I know the missing features will be added, so I regard this as a fully functional app in progress. It will surpass Pages in usability at some point unless Pages automates certain tasks like headers, contents, formatting, etc.