In this paper, we study the efficacy of login challenges at preventing account takeover, as well as evaluate the amount of friction these challenges create for normal users. These secondary authentication factors---presently deployed at Google, Microsoft, and other major identity providers as part of risk-aware authentication---trigger in response to a suspicious login or account recovery attempt. Using Google as a case study, we evaluate the effectiveness of fourteen device-based, delegation-based, knowledge-based, and resource-based challenges at preventing over 350,000 real-world hijacking attempts stemming from automated bots, phishers, and targeted attackers. We show that knowledge-based challenges prevent as few as 10% of hijacking attempts rooted in phishing and 73% of automated hijacking attempts. Device-based challenges provide the best protection, blocking over 94% of hijacking attempts rooted in phishing and 100% of automated hijacking attempts. We evaluate the usability limitations of each challenge based on a sample of 1.2M legitimate users. Our results illustrate that login challenges act as an important barrier to hijacking, but that friction in the process leads to 52% of legitimate users failing to sign-in---though 97% of users eventually access their account in a short period.