I've been using Write Monkey for all my first drafts, but then I discovered that unless I'm taking external notes while I write (to remember all those details), I end up making a hell of a lot of mistakes, forget a lot of things I wrote, and it makes the rewrite a lot harder.
Typically, I do keep a lot of hand-written notes in my notebook, but not all of it stays in one place or gets remembered. I'm using yWriter for nano this year and really, it has just enough bells and whistles for me to actually like it. Helps with consistency, you can make notes on any scene or chapter, general notes, and character and location profiles. The only thing I wish it had (like some other writing programs, I've forgotten their names atm, sorry) was a timeline.
I downloaded Scrivener, hoping to see an ability to put a timeline in there, but I didn't find it. Something about Scrivener just puts me off, whether it's the fugly corkboard (I'm a customizing geek :/ ) or the bubbly-looking notes on it, the wordpad look of it, just something about it turns me off. I can't get into Scrivener no matter how much I try and want to like it. Fortunately, yWriter is free, and I like its bells and whistles much better.
And yes, MS Word is for editing and rewrites. I love Word. The synonym finder and research tab is just way too distracting for writing a first draft, that's its only "downside."