This is the basis from where coop gaming was created.
Of course, the gameplay might have not translated very well into the 21st century, and most gamers today will only see its flaws, but back when Nintendo was at its strongest, anyone who had a console had this game. No FAQs, guides, Internet on how to beat levels, bosses, etc. Just trial and error. Mega Man fans will appreciate this as well. You can shoot an infinite amount of times in 8 directions, crouch, and jump. Weapon powerups are flimsy from the Fire upgrade to the awesome Spreader upgrade. Your character(s) are very vulnerable; only one hit from a shot or making physical contact with an enemy will kill you. You start with 3 lives and 2-3 continues, and that's it. It's a side-scrolling screen form left to right, and some levels are vertical scrolling (upwards) with a boss battle at the end. Only two levels have a different view behind your character, where you can only move left and right shooting away from the screen towards doors. After several rooms, a main door has to be opened, and a boss battle ensues.
Another thing is that if you have a powerup and are killed, you lose said powerup. There was nothing more frustrating than surviving the minefield level's first third with the Spreader to get shot at by a random sniper and having to face the tank with the regular bullets. If you've played this part at least once, you probably gave up and lost a life on those tanks if you didn't have an upgraded weapon. A weird side effect of this game: it made my button mashing so much faster. If you want to increase your button tapping rate, I recommend you trying this game.
The difficulty is based on how much you remember the levels, enemy positioning, etc. After hundreds of playthroughs, the game is not easy or hard: it just is. I've managed to get to a point where I beat the game by myself in 18 minutes, losing 3-4 lives at the most. I may or may not use the 30 lives code, depending on how "difficult" I want it to be. My personal best is going without dying in about 21 minutes and finishing the game.
I now play games with friends in XBox and PS3 and don't see very many games like this anymore, although there are a few that get very close (Gears of War trilogy is the first one that comes to mind).
I'm biased because I grew up playing this game, but it's one of the best games I've played. Sure, there are more complex coop games out there, but this one started it all, as well as being one of the first popular games that had a secret code (not so secret) to alter gameplay:
Up, Up, Down, Down, Left, Right, Left, Right, B, A, Start.