We always felt that using a relational database to power our service was a bit like trying to fit a square peg in a round hole. Every application is different but in our case, we would gladly trade off consistency for performance and schema flexibility. I am not saying that Microsoft SQL Server is slow, because it isn't, but it does impose a more formal relationship with your data and that wasn't the best fit for us.

We moved our whole data tier to MongoDB because we were not particularly interested in paying for more SQL Server licenses as we scaled up our operations.