Before even starting to prepare for this certification I felt pretty confident that I’m able to secure it without dropping a sweat. After passing the certification I felt that I have been completely wrong since the beginning =)
The reason for my confidence was that I contribute to spark-packages as well as to some Spark-related OSS projects (i.e. Apache Zeppelin), apart from the fact that I’m using Spark for more than a year now and I’ve been on the first Spark Summit and a number of Spark-related meetups in London. 
I thought that’d be more than enough just to skim a recommended “Learning Spark” book by one of the Spark’s creators before the exam.
I wish I had at least read the impressions of those who’d gone to the exam before me =)
I can definitely say that it’s one of the toughest IT certifications I’ve ever passed. O’reilly & Databricks have really put a lot of emphasis on practical experience that can’t be read anywhere and can’t be constituted without deep digging into the framework internals. I had no experience with Spark on YARN since I was using it mostly on Mesos cluster or in standalone mode, which made me drop a lot of sweat =)
If I had been looking for a new developer with Spark experience I would surely have given him a number of points if he has this certificate. It’s really based on practical experience, but not on theoretical knowledge which makes passing it valuable.