Even though it can get hard as hell, it is clearly worth playing again and again and again.
2D games have always been that special something for me and this is one game that really shows that utterly awesome games don't need that extra dimension. Even though this game has pretty much nothing apart from the story(talking about the lack of sidequests etc.), it's a truly well-built adventure game packed with loads and more loads of fun.
Practically the whole game is just about staying alive and killing everyone who comes in the way. On your way you'll find lots of different power ups/upgrades, and they will help you to reveal new passages or to beat enemies that you couldn't before. Saving happens through save rooms which you can find pretty much everywhere.
The game itself is a remake of a NES game with some additions, and what a god damn remake they managed to create. I myself have never played the nes version, though I have seen gameplay videos and photos and compared to the old version, this is a completely new game. It's somewhat similar to its GBA predecessor, Metroid Fusion, though at least in my opinion they made Fusion way too hard, and that problem was partly nullified when making Zero Mission.
Zero Mission follows a traditional adventure game concept that includes many different worlds that each have their own boss battles. After most boss battles you will gain a power up of some sort, like in Fusion, however there are some exceptions too. Health, as well as consumable weapons also have upgrades that simply boost the capacity, and those upgrades can be found a lot more often than those major power ups.
Due to the fact that Zero Mission is the most advanced GBA Metroid, it has earned my 10; Fusion lost to it when it comes to the rock-hard difficulty. Even though it's a pretty hard game, even a rookie to games like this should have the chance to try it out as it's truly worth the money. It's even worth playing again and again, and you still never get tired of it, trust me. Metroid is a great game series in my opinion too, and this one definitely keeps the reputation high.