I've used simultaneous initiative both with and without declaring actions. In my own fantasy game, I just tell everyone, your actions all happen at once, now, let's go around the table and handle your actions. Then I'll handle the opponents, and then if anyone is dead/unconscious, I handle that.

I recently ran a game of Daredevils (FGU), which also does simultaneous actions, but with announcements ahead of time. There are six types of actions, such as "Movement", "Observe", "Fire weapon", "Operate Vehicle". You put a die in front of you with the number of the action you want your character to do; then, when it comes your turn to handle your action, you choose an action according what type of action you announced. If you chose "Movement", you could choose "Full Move, Jump, Alter Position, or Disengage". If you chose "Operate Vehicle", then you should choose "Throw, Short Function, Hip Fire, Defend, Speak, Observe, Strike, or Drive".

This also worked very well; the basic announcement is simple, and then we go around the table dealing with the real action.