There is no way I can put down all I feel about this game in one post. Is this the best game ever? I don't know. But it's certainly in the top 5 NES games (a list which shifts and changes like mercury). It's not as well-known as other best-ofs like Excitebike, Castlevania, SMB 2 and 3, etc, but the fact is it is quite as good and frequently better than these games. Here's the gameplay premise: BC is a platform game in which you cannot jump - instead you are supplied with a bionic arm which you can shoot out in 5 directions. You can grapple onto platforms, stun enemies, swing around, or just do it for the sound. The commando of the title is a bit fragile, and while he can survive a fall from any height, a single bullet or even collision with an enemy will kill him (remember the days?). The truth is, though, he is an incredible badass, and you have to get through some probably 12 levels and 8 or so neutral zones to complete your quest - nothing less than the toppling of a new nazi empire.
The game in its original Japanese actually had Hitler in its name (Hitler no Fukkatsu) and swastikas adorning their official doors, banners, etc. This was deemed a little edgy for the 6-12 American child market, so the nazis were recast as the Badds, and Hitler as "Master-D." Interestingly, they did not remove his likeness or the incredibly violent end (wait for it!) to which he comes.
In any case, the gameplay is incredibly fun - the stages are very diverse, from the mountainous tower of area 5 to the the Tora Bora-like tunnel network of area 8. Furthermore, if your helicopter bumps into an enemy transport on the map, you are entered into an Ikari Warriors-like top-down action stage in which you must fight your way to the end. There are many ways to go through the game; some stages are entirely optional, while others are necessary and have prerequisites (flares for the underground level). You are rewarded with items on nearly every level - weapons, armor, items, new communicators (allowing you to view more hilarious engrish dialogue), and so on.
My brother and I used to play through it in one sitting all the time - there is no save or password system so you have to do it all at once. Now I play it through when I feel nostalgic at the coffee shop, and need to kill an hour or so. Bionic Commando is one of the best games on the NES, and holds a special place in my heart for being one of the few games I actually got good at, memorized, and beat as a kid... over and over and over again.
If you're a Bionic Commando fan already, you should check this site out for a trip: Bionic Commando Database