Use different characters and it won't be boring for you. Though I do agree that it doesn't have huge reply value. I went back to try to get all the secret coins and S ranks on all the level.
Again, I agree. Though there are some pretty sick combos once you know what the input commands are. I personally love Aokiji's Ice Age combo, pretty hax though.
Some of the NPCs should of been playable i'd rather have a gigant battle game. That was fun to play.