Pros
Welcome to the Brussels bubble: being part of making history, international atmosphere, invitations to events, email boxes full of newsletters and work filled with complexity and novelty. MEP office is a great place to see from within what is it like to drive policy-making in EU-institutions. You learn soft skills on how to behave accordingly in office environment, use proactivity to get things done, collaborate well with other colleagues, priorities your time and stay up-to-date in your field of work. It's also great if you are well aligned with your MEPs political opinions: nothing is more fun than writing political content that gets rubber stamped as it is.
Cons
Work was too easy, which did not provide that much room for development in terms of hard skills. The hierarchy and short-term contracts makes you an outsider from the actual 'game'. Working at a MEP office is pretty much exporting your own home country with you: if you are for example French, you read French newspapers, answers emails in French, eat lunch with other French offices, check French grammar for content writing and work with French colleagues. You have to put extra effort to create an international network. The pay is bad, around 1200e/month + 200€ reallocation benefit + daily allowances from one Strasbourg trips, but the job is interesting, fun and not too hectic.