I think the Final Smash's will be fairly useful but not so powerful that they destroy the gameplay. I trust Nintendo will get the balance right.
Also the website's description suggests that they don't necessarily have to be used immediately. However as previously stated, there will probably be an allocated time limit before the effect expires.
Also I for one wouldn't mind a short couple second long animation because although it will break up the action for a short while, it would put emphasis on the importance of the final smash.