In general, I'm a contented user. I wish I could use it more, but most of my printed writing is done with Word on an XP box. I use it for limited writing on OS; I've never really given it an acid test.
Over the years I've had two medium and one big issue with Nisus.
The first medium issue is that earlier versions didn't manage Word .DOC images well. Nisus would open the document, but when it saved the documented the images were not compressed. In particular, Nisus did not implement even PNG compression. I don't know if this is still a problem, but I've never seen mention of a fix.
The second medium issue is that even at NW Pro 1.2 you still can't save in Open Document Format.
The major issue is that the 10.5 update broke Nisus Writer Express and Nisus didn't provide a free fix. NWE was the only significant Mac app I use that didn't work in 10.5. It smells like Nisus was doing something programmatically fishy, and so they ought to have provided a free patch.
On the other hand Nisus uses RTF as a native file format, so Nisus documents are more likely than any other word-processing format to be readable in 10-15 years. The Style implementation (Pro version) is simple and excellent (aside: spit upon the Beast), and now that moving Navigator/TOC entries moves document blocks is a very reasonable outliner as well. It's the closest thing to an integrated wordpressor/outliner I've seen since FullWrite Professional. (MORE, bless its heart, was fundamentally an outliner -- though it was a better word processor than most.)
One of the strengths of OS X is that we have choices about word processors. Microsoft Word of course, but also Pages, Nisus Writer (Express and Pro), Mellel, and about 3-4 others (honorable mention to MORE's reincarnation - OmniOutliner). Nisus Writer's RTF support, style sheets, performance, and outliner features mean it's worth a close look by anyone who can avoid Word.