for my writing needs, I've looked at DevonThink, OmniOutliner and sundry other tools before reverting to Text Wrangler.
Plain text files (one for each chapter) are easy to use, Text Wrangler handles search and replace jobs easily, and when the MS is ready for sticking into Word or whatever that's no problem.
And I don't get distracted with extraneous features when I should just be writing.
Unleashing the Writer: Jer's Novel Writer