The learning from other players is a great advantage, especially since the federation has its own internal message board.
Another advantage is when the fed declares fed war, you can attack from +200% rather than the standard +130%.
Joker`s disadvantage is accurate. The other possible disadvantage is you cannot attack those in your fed, limiting available targets.
The benefits far outweigh the negatives.
To choose a fed, look at who is in it. You want a fed with players that have higher PR and planet count than you, as well as variety of races. That means you can learn from them. Also look for a fed that has mostly active players.