The author gave a great talk at Windy City Rails [1] on this topic. He talked about two domains - biology and comics - and showed how absurdly complex a SQL schema would be that modeled them rigorously. I hadn't really understood a practical case for schemaless databases, but his talk was helpful (and really funny).<p>[1] <a href="http://vimeo.com/6702444" rel="nofollow">http://vimeo.com/6702444</a>