Nisus Writer Pro is for those who need the features of apps like Word or Mellel, but want an easier to use interface, better typography and great, responsive support.
While I usually recommend Scrivener for outlining, researching and do large part of the writing of long (scientific) texts, Nisus is the TOP choice to make corrections, layout/formatting and finalise the work. It works nicely together with Scrivener, they both use RTF/RFTD, so that's about perfect.
For all those unhappy with the latest release of Apple Pages, who need a more powerful "classical" word processor, I too recommend Nisus Writer (for Pages' layout functions, Swift Publisher is a good alternative).
+ BEST SEARCH- and selection-functions, EVER. No other app comes close. With help of "markers" you can search for virtually anything. If you ever had to clean up a 450 pages PhD thesis you get off your chair and applaud seeing this! :-)
+ CORRECT HYPHENATION for other languages than English! This is where both Mellel and Pages fail – what can get really embarrassing… Nisus does it perfectly (hyphenation is editable using TeX as far as I understood, but it is correct, so I never had to tinker with it).
+ good typography. Nisus uses OS X's own text engine, therefore no ugly "white spots" between the words in justified alignment like you get in Word or Mellel, letters go smoothly one into the other, they aren't block wise displayed (and printed) like in the other two apps.
+ easy to use interface, everything customisable (create your own toolbar items, with custom icons, drawer's menus customisable etc.)
+ really good support of all common file types, great compatibility with MS Word (Nisus uses the LibreOffice translator)
+ very complete foreign language features, right-to-left, for every language the correct smart quotes (resp. guillemets, this is customisable too of course), and yes I've mentioned it, but again: correct hyphenation!
+ Cross references, footnotes over multiple pages and what else you need for scientific texts (bibliography apps integration, etc.)
+ integrated Nisus Thesaurus, very practical, if not essential when doing corrections.
+ Both portrait and landscape orientation can be set individually for each page/section in a file. Very practical if you have a larger table or figure in your text
+ smooth scrolling (unlike Word) and good performance also with very long texts (except the opening of the file: that can take quite some time with a large file)
+ efficient user interface: Nisus comes with its own macro script language so you can automise lots of your work
+ multiple clipboards with history
+ practical navigation sidebar (titles, bookmarks, etc.)
+ merge fields easy to use
+ best-in-class (my opinion) track changes and comments, compatible with Word
+ Great support, regular updates
Things that are not so great:
– the app looks a bit old, especially the icons are in need for a revamp (however you can replace them with your own icons). Not really shiny the whole thing.
– no footnotes in tables, you need to tinker there (write superscript numbers in the table, then at the end of the table insert a footnote and make its ref number invisible. It's OK but kind of sucks if you have a table over multiple pages). Nisus is working on that problem I heard…
– the QuickLook plugin for RTF and RTFD is made by Apple and isn't so great.