Objectives: Internships can bring a host of professional and academic benefits to students. Then, how do User Experience (UX) internships influence Human-Computer Interaction (HCI) graduate students' professional and academic growth? What are the challenges experienced by HCI graduate students during internships? We explored these two research questions. Participants: Our study participants were 42 HCI graduate students who completed UX internships. They came from computing and related disciplines, including computer science, information technology, psychology, and design. Some of the participants' internship titles were Interaction Designer, Design Researcher, UX Programmer, and Business Intelligence Analyst. Study Method: We conducted a thematic analysis on 42 graduate students' UX internship reports that were collected over 6 years to uncover themes in relation to our two research questions. Findings: As for UX internship benefits, we found that students learned about the workplace culture (e.g., academia vs. industry/government on research design processes) and core UX technical (e.g., research, design, programming) and people skills (e.g., teamwork, empathy toward end-users); they also realized what they wanted in future careers after completing their internships. We also found internship challenges that were related to the internship program (e.g., the availability of internship opportunities), the host organizations (e.g., the quality of mentorship received), and remote working (e.g., difficulty over conducting remote usability testing). Conclusions: We make practical recommendations for HCI educators, UX practitioners, and HCI graduate students on how they can work collaboratively to create a meaningful UX internship experience. These recommendations include researching the host organization prior to internships, providing comprehensive onboarding, and being transparent with internship constraints.