So I was wrong in my last review. This episode doesn't officially start the Impel Down arc of One Piece, but it does set it up well while pacing out a bit of amusing filler.
Saving Ace! is definitely a fun episode, but it is what it is - a bridge that connects Amazon Lily with Impel Down. Nothing much actually happens while Luffy learns more about Ace's impending execution and the true identity of Boa Hancock, but Luffy's song and dance routine is hilarious, and it's hard not to become envious when the women of Amazon Lily line up to play with Luffy's rubber body.
Overall, it's a fun filler episode that makes you hungry for the epic meat that is Impel Down. I still can't wait!