I used to love upper/lower/extra playfields but have found most of them typically get a really old over time (ACDC, FG, TF). When there is no risk of losing your ball it's tough to maintain the excitement and adrenaline. A few have been done well though, where they are quick to play, have a real benefit, or are truly integral and important in the game - TSPP comes to mind as one of THE BEST additional mini playfields on a game.