It's neat, often overlooked, but while it's impressive how much it does with very few concepts, by the end, it feels like it's overstayed its welcome, and every concept it's used has been stretched too thin. The bonus stages (and final boss) also feel very much at odds with the pace and playstyle of the rest of the game, requiring a speed and precision that the control scheme does not do much to accommodate. I would recommend against 100% completion (there's no bonus for it anyway). It would've been neat to see the idea explored again on the DS or 3DS, where the second screen could be used to see both aboveground and belowground simultaneously (a limitation that becomes an issue in a few stages with underground enemies), but it's too late for that now.