The Focus on Force study guides and practice exams helped me to understand how the questions are formatted (with some super tricky ones designed to trip you up), and fill in many knowledge gaps that I wasn't exposed to previously.
I successfully passed 5 certifications first time using their materials (Sales Cloud Consultant, Service Cloud Consultant, Platform App Builder, Advanced Admin, Admin). I love that thorough explanations and links to Salesforce sources are provided in each answer so that you can deeply understand the content.
Thanks Focus on Force for the great learning experience!