I'm happy outlining in Scrivener, as much as I outline. But some people like to outline big time. For them, perhaps, Scrivener is not enough. They need a dedicated outliner, such as OmniOutliner, or something of its ilk.

Can you do your brainstorming and something else then bring the results into Scrivener with little pain?

Oh yes. Provided your outlining program can export into the common outline file format OPML (which most, including OmniOutliner, can).

Here's <em>A Study in Scarlet </em>in OmniOutliner. This is a sample file from the developers themselves and includes the entire book (which messes up the export a little -- fiddle with where you put things to get the best results.

<a href="http://davidhewson.com/wp-content/uploads/2011/04/Screen-shot-2011-04-15-at-08.47.35.png"><img class="aligncenter size-large wp-image-8111" title="Screen shot 2011-04-15 at 08.47.35" src="http://davidhewson.com/wp-content/uploads/2011/04/Screen-shot-2011-04-15-at-08.47.35-640x637.png" alt="" width="635" height="632" /></a>

Export it as OPML.

<img class="aligncenter size-full wp-image-8112" title="Screen shot 2011-04-15 at 08.47.56" src="http://davidhewson.com/wp-content/uploads/2011/04/Screen-shot-2011-04-15-at-08.47.56.png" alt="" width="586" height="286" />

The create a new Scrivener document and simply import the OPML file. You'll get this.

<a href="http://davidhewson.com/wp-content/uploads/2011/04/Screen-shot-2011-04-15-at-09.03.30.png"><img class="aligncenter size-large wp-image-8114" title="Screen shot 2011-04-15 at 09.03.30" src="http://davidhewson.com/wp-content/uploads/2011/04/Screen-shot-2011-04-15-at-09.03.30-640x509.png" alt="" width="635" height="505" /></a>

Note that the body text is used for the title of the scene (which in the case of this sample document is the entire chapter of the book). You will want to do something different so play with the options. I can't see a way to fill in other parts of the Scrivener document such as the synopsis through OPML. You're basically talking