TL;DR I passed. :P Here's a run-down of what the experience was like.
Up-front disclaimer: I work for Acquia. However, I was not involved in the creation of Acquia Certification and had no insight to the test beforehand apart from a single sample question. I'm also writing this on my personal blog, rather than Acquia.com, because these are *my* thoughts/impressions, and haven't been vetted by anyone else. :) Carry on!