Metroid: Zero Mission is a good game to introduce to those that are new to the franchise. Thanks to the ability to set the difficult level, well designed stages, and beautiful graphics, there seems to be a lower rate of frustration compared to the other games.
In terms of extras, there isn't much, but what it has is well worth it; after completing the game, you can play the original NES Metroid. Considering you can buy it from the Nintendo Classics line for $20 makes Zero Mission a very good deal.
However, don't expect as much storytelling as the Prime series… actually don't expect any at all. Aside from one cool cut scene (hint: space pirates) you pretty much explain the story of this game in fewer words than this review.
In short, this game is highly recommended in any gaming library.