Pros
The CEA is in average a decent research institute. Like all big research center it is possible to find very good group but also very bad ones. Colleagues are in general great, but it is better if you speak French.
Cons
People behave differently with you if you have a short term contract (CDD) or a long term contract (CDI). This honestly belong to the past, I am used to judge people by experience and accomplishment rather than by the job title. While some research areas are stimulating, it is full of people that stays just because their job is untouchable, having a CDI contract. In this way, real talents will leave also if they are offered a CDI contract. Although CEA is claiming to be an international excellence, I am not sure it is possible to have a career progression without speaking French.