National Science Foundation (NSF) funded Research Experiences for Undergraduates (REUs) are explicitly intended to reach minoritized students in STEM and those who have few research opportunities. Many undergraduates are encouraged to seek them out, but their actual efficacy is not well-established, and the out-of-state travel required for many attendees may prove a significant barrier for the very students REUs wish to reach. We interviewed physics graduate students who attended REUs as undergrauates, focusing on how the REUs benefitted them, barriers they faced attending REUs, and their relationship with their REU mentors. Interviewees reported benefits that aligned with the NSF goals: skills, enculturation, and knowledge they had not received in their undergraduate institutions. They also reported financial barriers they faced which they were able to overcome due to their financial privilege. Participants also reported widely varying experiences with their mentors. Some mentors did and some did not meet their mentees where they were at in their career and skill levels. Some students did not know how to approach their mentors with their questions or needs.