If you haven't played this game yet, I feel sorry for you.
The gameplay is ridiculously responsive and it doesn't slow down even in multiplayer mode. It's a good thing too because you'll need that dexterity to beat all 8 stages. This is one challenging game. I'd say it's almost as difficult as 'Battletoads' but much more enjoyable because the level-designers have thrown in a generous number of items to help you out instead of just adding more and more things to kill you. For example, in the first stage there are 2 'S' power-ups(the best weapon around).
The 2nd and 4th stages are a little strange, but no less fun than the others. They use a single-point perspective that you will instantly recognize from old-school adventure games, but they keep the same gameplay. So basically you just run back and forth with lots of jumping and ducking until everything is destroyed and then you move on to the next fixed screen. It's too bad the hardware didn't exist at the time to support more interesting innovation.
If you run out of lives when you die--and you will die, my friend, you will-- you'll be pleased to find that you can take your buddy's extra lives just as if they were your own! If that seems underhanded you can always try to earn more beforehand by scoring more points, but there's no guarantee that your partner won't steal YOUR lives after you worked hard to get them. Either way, make good use of them. They're in short supply.
Rather than bickering over extra lives, your time is better spent fighting the real enemies in the game. No, i'm not talking about the guerrillas in the bush or even those scorpion-looking aliens. I'm talking about the 'F' items. They masquerade as power-ups but be careful not to collect them unless you enjoy being 'F'ed over by a weapon that moves at the break-neck speed of glacier-melt. I get the feeling they only left this one in the final version to make the other weapons look better by comparison.
The Bottom Line
All in all, this is one of the best games made for the NES so go pick it up, pick up a friend, and pick up those controllers already!