You know when you have a special case. If you don't - just the most convenient off the shelf database and get back to adding core functionality. It sounds like

You know when you have a special case. If you don't - just the most convenient off the shelf database and get back to adding core functionality. It sounds like you do, however I think that there are a lot of people who have no real reason to move away from SQL other than fashion. The database choice should be just like any other engineering decision - often your familiarity with the new tool is very important.