Browser-based blocking extensions such as Ad blockers and Tracker blockers have provisions that allow users to counter online tracking. While prior research has shown that these extensions suffer from several usability issues, we know little about real world blocking extension use, why users choose to adopt these extensions, and how effectively these extensions protect users against online tracking. To study these questions, we conducted two online surveys examining both users and non-users of blocking extensions. We have three main findings. First, we show both users and non-users of these extensions only possess a basic understanding of online tracking, and that participants' mental models only weakly relate with their behavior to adopt these extensions. Second, we find that that each type of blocking extension has a specific primary use associated with it. Finally, we find that users report that extensions only rarely break websites. However when websites break, users only disable their extensions if they trust and are familiar with the website. Based on our findings, we make recommendations for designing better protections against online tracking and outline directions for future work.