Pros
As a trainee, the European Commission offers to you an enormous opportunity to specialise in a policy sector and to gain valuable understanding of how the EU and the Commission works. Depending on your work ethic, trainees are left to choose the level of depth of their exposure to the policy area they are interested in and a little enthusiasm pays off with many opportunities to learn and work more, for as a trainee time is also short. The balance between work and social life is great and the traineeship is a great way to make new friends and have one of the best chapters of your life, without a doubt.
Cons
too short. sometimes you are in a unit that does not really seem to use its trainees very much, other times it's just your supervisor who is not that demanding/interesting. don't let this stop you, time is short, if you don't think you're going to learn much, ask to move.