Druid's main value add to the data infrastructure space is around power user-facing data applications at scale. The queries it is best at are OLAP/business intelligence style queries. It isn't really designed to be a general processing tool such as Hadoop or Spark. The open source data space is very complex, and there are many different solutions targeted towards many different use cases. Druid is better than other solutions at some of these use cases, and worse than other solutions at others.

I wrote my interpretation of the current open source data landscape here for anyone interested: http://imply.io/post/2015/11/04/big-data-zoo.html