Metroid and Action/adventure alike will find a solid game in Metroid II while others might find it too confusing.
Metroid II takes place a few months after the original metroid. After the Zebes incident, troops are sent to SR-388 to whipe out the Metroid race so they cannot harm the universe again. Unfortunatly, they don't return so the federation decides to send Samus Aran, who's allready familiar with the Metroids from her previous encounter, to SR-388. So basicly, the goal of the game is to destroy every single Metroid. The game consists of what fans like to call Metroid Cities, big areas which contain a handfull of Metroids. Once you clear that one, another becomes available after a huge earthquake, which will lower the lava found in the "Main hall" of the planet. Because of this, Metroid II is a linear game, though this works quite well in the context of the game. You'll also get quite a few upgrades along the way while you're at it.
As you might have noticed, I marked the game being hard. This is because of two factors. First of all, you don't have any kind of map in Metroid II. You'll get lost quite a few times making the game needlessly frustrating at times. The other factor are the metroids themselves. These creatures evolve and become more advanced as you move on, becoming more tougher as you progress. They will also often be found in enviroments that are to your disadventage, making it quite tough to hit them. Practice is neccesary, because the later Metroids can soak up to 50 missiles before they are finally defeated. Purists will get a kick out of this, others might find it too intense.
Metroid II's graphics are very good at gameboy standards. They're rich, detailed enviroments, limited only by the screens small size. Samus herself also looks better then her NES incarnation. The Audio in the game is less impressive, in fact, most of the times there is hardly any to be found. The game begins quite nice with a theme that's quite catchy but from teh moment you set steps in the first Metroid City, the music drops, and you'll often hear weird noices. This actually brilliantly makes Metroid II creepy at times, but that doesn't make the audio good. The sound effects are better though the weapons lack any kind of powerfull feel because of the silent sound effects.
This all doesn't keep Metroid II from being yet another brilliant Metroid game. It's a long, entertaining and fresh experience that you'll play over again at least a few more times after you've beaten it the first time (which won't be easy by any stretch of the imagination, though not as hard as, let's say, Devil May Cry 3). It's too bad that Metroid II doesn't have anything else to offer then that, since it deffenitivly would've added to the experience.
All in all, Metroid II doesn't dissapoint, even considering its (few) flaws. If you are a metroid fan, go get it. If you are an action/adventure game fan, go get it as well. Others might want to try Super Metroid instead first, but even those will enjoy what Metroid II has to offer.