Amazon also provides tools that convert unprotected ePub (both KindleGen and Kindle Previewer will do this):
http://www.amazon.com/gp/feature.html?docId=1000234621
Kindle Previewer in particular will show you exactly how it will actually look on a Kindle, which is kind of nice.
I haven't compared the results with calibre's in any systematic way, and calibre offers options that may be useful, but I like to have a lot of tools to choose from.