I got the results of my exam today (after having to wait for almost 2 weeks after the actual exam) and got the "passed" result.
Other than others who wrote here I already had several years of experience working with GCP in the web development area (App Engine, Firebase, Cloud Functions, CDN, etc.). But that alone wouldn’t have been sufficient to pass the exam.
Having done other preparation courses for certifications I think this course by Matthias is one of the better one and gives a pretty solid foundation while putting focus on the important stuff. I can also strongly approve Mattias when he emphasized how important understanding the data flows is. I think the Google exam is actually better at testing that you really understood key concepts and that you had worked with GCP by giving you more or less complex scenarios in the exam for which you have to evaluate the proper solutions.
What I did for those contexts I did not feel fully comfortable with was to play around in the Cloud Playground a lot. Just giving yourself some goal to achieve by mirroring a simple setup I knew from some work related background and try to make it work in GCP. e.g. I want to host some publicly accessible web application (using App Engine) in a DMZ which should access a load balanced background service (e.g. on GCE) in a network, which is not publicly accessible from the internet. By trying to make this things work you learn a lot about IAM, default service role permissions, load balancing options, etc. and you get more confidence using this services in real world contexts.
Another word on the ACE practice exam: From my perception this exam gives you a pretty good feeling what kind of questions you might expect. But when comparing it with the exam I took, the distribution of topics was way of. I almost had no questions regarding GKE and exactly 0 question for which I actually needed Kubernetes know-how (or how to use kubectl). On the other hand side I got a bunch of questions regarding network and access security, default permission, especially on user and service accounts and IAM roles.
A big thanks to Mattias and the ACG Team for this well prepared course.