Passkeys were introduced to mitigate passwordrelated security risks. However, passwords remain prevalent, and the usability of synchronization, one of the core benefits of passkeys, has not yet been studied. To address this gap, we conducted a large-scale field study examining passkey adoption and usage by partnering with an industrial healthcare service. In Phase 1(n=5,057), users chose between a passkey and a password with 2FA during registration. We reveal that (especially inexperienced) users struggle with passkey adoption but still perceive passkeys to offer superior usability and acceptance. Our survey results identified novel reasons why users decline passkeys, notably the habit of using passwords and lack of passkey experience, and pinpointed adoption barriers, including new misconceptions and manual passkey activation. In Phase 2(n=2,716), we monitored authentication behavior over six months and captured immediate feedback through surveys. We found that passkey usability and acceptance continued to increase, and success rates surpassed the declining metrics of passwords. Still, we uncovered new issues with crossplatform synchronization, passkey sharing, and interactions with password managers. Finally, we provide recommendations to streamline the adoption and usability of passkeys.