All in all, Metroid Zero Mission is a must-have for Metroid fans and a great introduction for newcomers to the series.
Also, expect a few surprises after you torch Mother Brain. You'll find some new stealth-action gameplay elements that I feel were well executed and do much to freshen up the gameplay.
For the most part the game is also very easy. I can count on one hand the number of times that I died or was truly in danger of dieing before the last 1/4 of the game. I wish that Hard Mode had been unlocked by default or was the standard difficulty of play, although I'm sure some will find MZM's ease of play refreshing after Metroid Fusion's punishing difficulty level.
All in all, Metroid Zero Mission is a must-have for Metroid fans and an excellent introduction to the series for those who have never played it before.
My only true gripe is the length of the game. On my first play through, I obtained 74% completion in 4:04. Now, I chalk that up a little to my well honed Metroid-Sense™, but even newbies to the series aren't likely to take much more time than that on the first play through. To give the game credit, beating the game unlocks the original NES Metroid (which most will only try out just for kicks) and there are 8 different ending graphics, depending on how fast you beat the game and how many of the items you collect. Once you've obtained an ending image, you can view it any time.