Pros
Fast-paced working environment that offers many opportunities for how to learn new skills, deepen existing skillsets, and take on interesting projects to advance one's career in the sciences. The scientists and engineers at CERN perform cutting-edge research work, and working at CERN offers the opportunity to interface with collaborators from all over the world.
Cons
The CERN environment is very intense, and it is not easy to establish a work/life balance, especially for graduate students. Depending on the university and the PhD advisor, graduate students in physics often suffer poor mental and emotional wellbeing due to the high-volume of work/time expected of them. While these cons are not necessarily due to the CERN staff themselves, they work closely with institutes at the US and around the world that employ these students. PhD students and postdocs form the majority of the personpower that runs these experiments and often are working completely within the CERN environment.