Like Fusion before it, Zero Mission is far too short. That doesn't change that it's spectacular while it lasts.
The Bad: The game's way too short, just like Fusion; the metroids, Mother Brain and the Zero Suit ramp up the difficulty very suddenly; Metroid II didn't get the same treatment as this one.
I've said it before and I'll say it again. Metroid is one of the most fantastic, screwed over game series ever.
The original version of this game was on the NES, and used a password system, like plenty of other games of its era. But because it came out alongside The Legend of Zelda, which invented both the bird's eye view camera and the save system, it was eclipsed. It's not any fault of its own that Zelda came along when it did, and thus, it sold considerably less.
Other than Metroid Prime, this has been the case with every other Metroid in some facet. Metroid II is still monochrome, Super Metroid came out late so it got half the attention it deserved, and Fusion competed with A Link to the Past and its big brother, Prime, in 2002.
Now this classic has a better chance to shine than ever. Given the control system of Metroid Fusion, graphics exponentially better than the original version, a score of a more modern quality and a new huge chunk of gameplay, this is the definitive version of the first Metroid, hands down. It's not as difficult and uses a save system, so that may not appeal to absolute purists, but Metroid, in my mind, has been revived spectacularly.
The original story is the same as ever. Go to Zebes and get rid of the metroids, the Space Pirates, and the big bosses all over the place. It's a blast to play it with GBA level graphics and more enjoyable to the average present day gamer.
The new content is an interesting twist. After the big fight with Mother Brain Samus's suit loses its power and quits working, and she crash lands on the Space Pirate homeworld. The zero suit doesn't protect her from attacks like her power suit does, so the difficulty gets much higher from then on. That might be a bit of a snag to the extra content (although it did lengthen the game), but it doesn't mean the new content is no good.
Samus has to avoid the Space Pirates instead of fight them, as you'll quickly learn, but it does eventually lead to a power suit that's better than ever, and some more fantastic battles. Again, the new content isn't bad, it's just more difficult.
But despite that new content (and the fact you can play the original iteration of Metroid after beating it), the game remains too short. It's not as bad an issue as it was for Metroid Fusion, but it's not half as long as Link to the Past.
Is it worth it? Nowadays, it certainly is for anyone who just loves Metroid and wants a good collection of the games. It's certain to be cheap, and despite its length, the game is extremely fun the whole way through.