Having personally used the InDesign plugin for Kindle, I can honestly say it’s one of the easiest ways to create a Kindle-ready file. It’s literally just a few clicks from an .indd document to a Kindle book. You select “Export to Kindle”, pick a TOC option and a cover image, and save the file. Simple as that.
The plugin is best suited for books that are comprised of text (it’s perfect for novels) and other linear content. (if there are occasional graphics, that’s ok too). And while most formatting is easily preserved with character and paragraph stylesheets in the InDesign file, things like custom fonts aren’t yet supported. The best way to optimize your book files for Kindle is to convert a document with the plugin and then view it with the free Kindle Previewer, which provides a one-to-one experience with what your readers will see.
You can get the plug in here.
Do you know what just occurred to me? A few days ago Mike Cane wrote a post called "Amazon declares war on All Fronts". This plug in was another volley of fire at the Epub crowd.